Single electron transistor device

ABSTRACT

A transistor device is provided that includes a gate electrode disposed between source and drain electrodes and overlying a quantum dot structure realized by a modulation doped quantum well structure. A potential barrier surrounds the quantum dot structure. The transistor device can be configured for operation as a single electron transistor by means for biasing the gate and source electrodes to allow for tunneling of a single electron from the source electrode through the potential barrier surrounding the quantum dot structure and into the quantum dot structure, and means for biasing the gate and drain electrodes to allow for selective tunneling of a single electron from the quantum dot structure through the potential barrier surrounding the quantum dot structure to the drain electrode, wherein the selective tunneling of the single electron is based upon spin state of the single electron.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of Ser. No. 13/921,311, filedon Jun. 19, 2013, which is a continuation-in-part of Intl. Patent Appl.No. PCT/US12/51265, entitled “OPTICAL CLOSED LOOP MICRORESONATOR ANDTHYRISTOR MEMORY DEVICE” filed on Aug. 17, 2012, herein incorporated byreference in its entirety.

BACKGROUND

1. Field

The present application relates to semiconductor integrated circuitsthat implement a variety optoelectronic functions (such as opticalemitters, optical detectors, optical modulators, optical amplifiers, andoptical switches) and electronic functions (such as heterojunction fieldeffect transistors and bipolar field effect transistors).

2. State of the Art

The present application builds upon technology (referred to by theApplicant as “Planar Optoelectronic Technology” or “POET”) that providesfor the realization of a variety of devices (optoelectronic devices,logic circuits and/or signal processing circuits) utilizing inversionquantum-well channel device structures as described in detail in U.S.Pat. No. 6,031,243; U.S. patent application Ser. No. 09/556,285, filedon Apr. 24, 2000; U.S. patent application Ser. No. 09/798,316, filed onMar. 2, 2001; International Application No. PCT/US02/06802 filed on Mar.4, 2002; U.S. patent application Ser. No. 08/949,504, filed on Oct. 14,1997, U.S. patent application Ser. No. 10/200,967, filed on Jul. 23,2002; U.S. application Ser. No. 09/710,217, filed on Nov. 10, 2000; U.S.Patent Application No. 60/376,238, filed on Apr. 26, 2002; U.S. patentapplication Ser. No. 10/323,390, filed on Dec. 19, 2002; U.S. patentapplication Ser. No. 10/280,892, filed on Oct. 25, 2002; U.S. patentapplication Ser. No. 10/323,390, filed on Dec. 19, 2002; U.S. patentapplication Ser. No. 10/323,513, filed on Dec. 19, 2002; U.S. patentapplication Ser. No. 10/323,389, filed on Dec. 19, 2002; U.S. patentapplication Ser. No. 10/323,388, filed on Dec. 19, 2002; U.S. patentapplication Ser. No. 10/340,942, filed on Jan. 13, 2003; all of whichare hereby incorporated by reference in their entireties.

With these structures, a fabrication sequence can be used to make thedevices on a common substrate. In other words, n type and p typecontacts, critical etches, etc. can be used to realize all of thesedevices simultaneously on a common substrate. The essential features ofthis device structure include 1) an n-type modulation doped interfaceand a p-type modulation doped quantum well interface, 2) self-alignedn-type and p-type channel contacts formed by ion implantation, 3) n-typemetal contacts to the n-type ion implants and the bottom n-type layerstructure, and 4) p-type metal contacts to the p-type ion implants andthe top p-type layer structure. The active device structures arepreferably realized with a material system of group III-V materials(such as a GaAs/AlGaAs).

POET can be used to construct a variety of optoelectronic devices. POETcan also be used to construct a variety of high performance transistordevices, such as complementary NHFET and PHFET unipolar devices as wellas n-type and p-type HBT bipolar devices.

SUMMARY

A semiconductor device includes a plurality of semiconductor layerssupported on a substrate. The plurality of semiconductor layers includeat least one modulation doped quantum well (QW) structure offset from aquantum dot in quantum well (QD-in-QW) structure. The modulation dopedQW structure includes a charge sheet spaced from at least one QW by aspacer layer. The QD-in-QW structure has quantum dots (QDs) embedded inone or more QWs. The QDs are sized to support optical functions(emission, amplification, absorption) of electromagnetic radiation at acharacteristic wavelength.

In one embodiment, the QD-in-QW structure can include at least onetemplate/emission substructure pair separated by a barrier layer, thetemplate substructure can have smaller size QDs than the emissionsubstructure. Furthermore, the template substructure can define QDsembedded in a digital-graded quantum well, and the emission substructurecan define QDs embedded in an analog-graded quantum well.

In another embodiment, the QD-in-QW structure can define QDs embedded inat least one analog-graded quantum well.

In yet another embodiment, the plurality of semiconductor layers caninclude a plurality of QD-in-QW structures with QDs sized to supportoptical functions (emission, amplification, absorption) ofelectromagnetic radiation of different characteristic wavelengths (suchas optical wavelengths in range from 1300 nm to 1550 nm).

In still another embodiment, the QD-in-QW structure can be disposedopposite the charge sheet of the modulation doped QW structure andoffset from the at least one quantum well of the modulation doped QWstructure by a spacer layer. The spacer layer can have a thickness inthe range of 300-500 Å.

The semiconductor device can realize an integrated circuit including awide variety of optoelectronic devices that perform optical functions(emission, amplification, absorption) for electromagnetic radiation atthe characteristic wavelength(s) supported by the QDs of the QD-in-QWstructure(s) as well as including electrical transistors forconfiguration of such optoelectronic devices, logic circuitry and signalprocessing circuitry as needed.

Advantageously, the QDs embedded within the QD-in-QW structure(s) of theoptoelectronic device improve the efficiency of such optoelectronicdevices and integrated circuits based thereon. Specifically, thepopulation inversion necessary for emission and amplification as well asthe photon absorption mechanism for necessary for optical detectionoccurs more efficiently with the introduction of the quantum dots andthus decreases the necessary current required for lasing action andamplification and increases the photocurrent produced by absorption.Furthermore, the size of the embedded QDs can be controlled to dictatethe wavelength of the desired optical function (emission, amplificationand absorption).

In another aspect, a transistor device is provided with a gate terminalelectrode disposed between a source terminal electrode and a drainterminal electrode. The gate terminal electrode overlies a quantum dotstructure realized by a modulation doped quantum well structureincluding a charge sheet offset from at least one quantum well. Apotential barrier surrounds the quantum dot structure. The potentialbarrier can be defined by an ion implant region that surrounds the QDstructure. In one embodiment, the ion implant region is formed from anion species that reacts with the charge sheet of the modulation dopedquantum well structure under predefined high temperature conditions. Thelocation and dimensions of the potential barrier can be dictated by thelocation and size of an opening defined by a photomask that allows forthe implantation of the ion species. The ion species can include oxygenions.

The transistor device can be configured for operation as a singleelectron transistor. In this configuration, the transistor deviceincludes means for biasing the gate and source terminal electrodes toallow for tunneling of a single electron from the source terminalelectrode through the potential barrier surrounding the quantum dotstructure and into the quantum dot structure, and means for biasing thegate and drain terminal electrodes to allow for selective tunneling of asingle electron from the quantum dot structure through the potentialbarrier surrounding the quantum dot structure to the drain terminalelectrode, wherein the selective tunneling of the single electron isbased upon spin state of the single electron.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary optoelectronicintegrated circuit device structure in accordance with the presentapplication, with an n-channel HFET device and/or p-channel HFET deviceincluded therein.

FIG. 2 is an exemplary current-voltage characteristic curve for ann-channel HFET device realized by the optoelectronic integrated circuitdevice structure of FIG. 1.

FIG. 3 is a schematic illustration of the exemplary optoelectronicintegrated circuit device structure of the present application with ann-channel base BICFET device or a p-channel base BICFET device includedtherein.

FIG. 4 is an exemplary current-voltage characteristic curve for ann-channel base BICFET device realized from the optoelectronic integratedcircuit device structure of FIG. 3.

FIG. 5 is a schematic illustration of the exemplary optoelectronicintegrated circuit device structure of the present application with aquantum well laser or detector device included therein.

FIG. 6 is an exemplary energy band diagram showing the currentsgenerated during lasing operation of a quantum well laser realized fromthe optoelectronic integrated circuit device structure of FIG. 5.

FIG. 7 is an exemplary current-voltage characteristic curve for aquantum well laser realized from the optoelectronic integrated circuitdevice structure of FIG. 5.

FIG. 8 is a schematic illustration of the exemplary optoelectronicintegrated circuit device structure of the present application with adifferent quantum well laser or detector device included therein.

FIG. 9 is a schematic illustration of the exemplary optoelectronicintegrated circuit device structure of the present application with aquantum well thyristor device included therein.

FIG. 10 is an exemplary current-voltage characteristic curve for aquantum well thyristor realized from the optoelectronic integratedcircuit device structure of FIG. 9.

FIGS. 11A and 11B are schematic illustrations of exemplary layerstructures for the QD-in-QW structures (24, 28) of the optoelectronicintegrated circuit device structures of FIGS. 1 to 10.

FIGS. 12A to 12C, collectively, are a chart illustrating an exemplarylayer structure for realizing the optoelectronic integrated circuitdevice structures of FIGS. 1 to 11B.

FIG. 13 is a schematic illustration of an exemplary n-channel HFETdevice realized as part of an optoelectronic integrated circuit thatemploys the layer structure of FIGS. 12A to 12C.

FIG. 14 is a schematic illustration of an exemplary n-channel baseBICFET device realized as part of an optoelectronic integrated circuitthat employs the layer structure of FIGS. 12A to 12C.

FIGS. 15A-15C are schematic illustrations of an exemplary quantum welllaser or detector device realized as part of an optoelectronicintegrated circuit that employs the layer structure of FIGS. 12A to 12C;FIG. 15A is a perspective schematic view of the passive waveguidestructure and the quantum well laser or detector device; FIG. 15B is across-sectional schematic view of the quantum well laser or detectordevice through the cross-section labeled 15B-15B in FIG. 15A; FIG. 15Cis a cross-sectional schematic view of the passive waveguide structurethrough the cross-section labeled 15C-15C in FIG. 15A.

FIG. 16 is a schematic illustration of a different quantum well laser ordetector device realized as part of an optoelectronic integrated circuitthat employs the layer structure of FIGS. 12A to 12C.

FIG. 17 is a schematic illustration of an exemplary quantum wellthyristor device realized as part of an optoelectronic integratedcircuit that employs the layer structure of FIGS. 12A to 12C.

FIGS. 18A-18F are schematic illustrations of an exemplary closed-loopmicroresonator realized as part of an optoelectronic integrated circuitthat employs the layer structure of FIGS. 12A to 12C; FIG. 18A is aperspective schematic view of the closed-loop microresonator; FIG. 18Bis a top schematic view of the closed-loop microresonator; FIG. 18C is across-sectional schematic view of the closed-loop microresonator throughthe cross-section labeled 18C-18C in FIG. 18B; FIG. 18D is across-sectional schematic view of the closed-loop microresonator throughthe cross-section labeled 18D-18D in FIG. 18B; FIG. 18E is a perspectiveschematic view of the closed-loop microresonator fabricated alongside aheater transistor device for wavelength tuning; and FIG. 18F is aperspective schematic view of the closed-loop microresonator fabricatedalongside a set of evanescently coupled closed-loop waveguide structuresfor wavelength tuning.

FIGS. 19A-19C are schematic illustrations of an exemplary waveguideoptical coupler realized as part of an optoelectronic integrated circuitthat employs the layer structure of FIGS. 12A to 12C; FIG. 19A is a topschematic view of the waveguide optical coupler; FIG. 19B is across-sectional schematic view of the waveguide optical coupler throughthe cross-section labeled 19B-19B in FIG. 19A; FIG. 19C is across-sectional schematic view of the waveguide optical coupler throughthe cross-section labeled 19C-19C in FIG. 19A.

FIGS. 20A-20C are schematic illustrations of an exemplary waveguideoptical amplifier realized as part of an optoelectronic integratedcircuit that employs the layer structure of FIGS. 12A to 12C; FIG. 20Ais a perspective schematic view of the waveguide optical amplifier andassociated passive waveguides for guiding light into and out of thewaveguide optical amplifier; FIG. 20B is a cross-sectional schematicview of the waveguide optical amplifier through the cross-sectionlabeled 20B-20B in FIG. 20A; and FIG. 20C is a cross-sectional schematicview of the passive waveguide through the cross-section labeled 20C-20Cin FIG. 20A.

FIGS. 21A and 21B are a schematic cross-sectional view and top view,respectively, of an exemplary single electron transistor device inaccordance with the present application.

FIG. 22A is a schematic illustration of the load, compute and readoperations of the exemplary single electron transistor of FIGS. 21A and22B.

FIG. 22B are exemplary voltage signal waveforms that can be applied tothe gate, source and drain terminal electrodes of the single electrontransistor device to carry out the load, compute and read operations ofFIG. 22A.

FIG. 22C are generalizations of current that flows into the quantum dotand read out from the quantum dot of the single electron transistorduring the load, compute, and read operations, respectively of FIG. 22A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to FIG. 1, the device structure of the present applicationincludes bottom dielectric distributed bragg reflector (DBR) mirror 12formed on substrate 10. The bottom DBR mirror 12 is typically formed bydepositing pairs of semiconductor or dielectric materials with differentrefractive indices. When two materials with different refractive indicesare placed together to form a junction, light will be reflected at thejunction. The amount of light reflected at one such boundary is small.However, if multiple junctions/layer pairs are stacked periodically witheach layer having a quarter-wave (λ/4) optical thickness, thereflections from each of the boundaries will be added in phase toproduce a large amount of reflected light (e.g., a large reflectioncoefficient) at the particular center wavelength λ_(D). Deposited uponthe bottom DBR mirror 12 is the active device structure suitable forrealizing complementary heterostructure field-effect transistor (HFET)devices. The first of these complementary HFET devices is a p-channelHFET which has a p-type modulation doped quantum well (QW) structure 20with an n-type gate region (i.e., n-type ohmic contact layer 14 andn-type layer(s) 16)) below the p-type modulation doped QW structure 20.A QD-In-QW structure 22 is formed above the p-type modulation doped QWstructure 20. The QD-In-QW structure 24 includes at least one QW layerwith self-assembled quantum dots (QDs) embedded therein. The QD-In-QWstructure 24 is spaced from the QW(s) of the p-type modulation doped QWstructure 20 by an undoped spacer layer 22. The second of thesecomplementary HFET devices is an n-channel HFET which includes an n-typemodulation doped QW structure 32 with a p-type gate region (i.e., p-typelayer(s) 36 and p-type ohmic contact 38) formed above the n-typemodulation doped QW structure 32. A QD-In-QW structure 28 is formedbelow the n-type modulation doped QW structure 32. The QD-In-QWstructure 28 includes at least one QW layer with self-assembled quantumdots (QDs) embedded therein. The QD-In-QW structure 28 is spaced fromthe QWs of the n-type modulation doped QW structure 20 by an undopedspacer layer 30. The QD-In-QW structure 28 is formed above one or morespacer layers 26 that interface to the QD-in-QW structure 24. The layersencompassing the spacer layer 30, the QD-in-QW structure 28, the spacerlayer(s) 26, the QD-in-QW structure 24, the spacer layer 22, and thep-type modulation doped QW structure 20 forms the collector region ofthe n-channel HFET. The layers encompassing the spacer layer 22, theQD-in-QW structure 24, the spacer layer(s) 26, the QD-in-QW structure28, the spacer layer 30, and the n-type modulation doped QW structure 32forms the collector region of the p-channel HFET. Such collector regionsare analogous to the substrate region of a MOSFET device as is wellknown. Therefore a non-inverted n-channel HFET device is stacked upon aninverted p-channel HFET device as part of the active device structure.

The active device layer structure begins with n-type ohmic contactlayer(s) 14 which enables the formation of ohmic contacts thereto.Deposited on layer 14 are one or more n-type layers 16 and an undopedspacer layer 18 which serve electrically as part of the gate of thep-channel HFET device and optically as a part of the lower waveguidecladding of the device. Deposited on layer 18 is the p-type modulationdoped QW structure 20 that defines a p-type charge sheet offset from oneor more QWs (which may be formed from strained or unstrainedheterojunction materials) by an undoped spacer layer. The p-type chargesheet is formed first below the undoped spacer and the one or more QWsof the p-type modulation doped QW structure 20. All of the layers grownthus far form the p-channel HFET device with the gate ohmic contact onthe bottom. Deposited on the p-type modulation doped QW structure 20 isan undoped spacer layer 22 followed by the QD-In-QW structure 24 (whichincludes at least one QW layer with self-assembled QDs embeddedtherein). The undoped spacer layer 22 provides an offset between theQW(s) of the p-type modulation doped QW structure 20 and the QD-In-QWstructure 24.

Deposited on the QD-In-QW structure 24 is the spacer layer(s) 26followed by the QD-in-QW structure 28, the undoped spacer layer 30 andthe n-type modulation doped QW structure 32. The n-type modulation dopedQW structure 32 defines an n-type charge sheet offset from one or moreQWs by an undoped spacer layer. The n-type charge sheet is formed lastabove the undoped spacer and the one or more QWs of the n-typemodulation doped QW structure 32. The undoped spacer layer 30 providesan offset between the QD-In-QW structure 28 and the QW(s) of the b-typemodulation doped QW structure 32.

Deposited on the n-type modulation doped QW structure 32 is an undopedspacer layer 34 and one or more p-type layers 36 which can serveelectrically as part of the gate of the n-channel HFET and optically aspart of the upper waveguide cladding of the device. Preferably, thep-type layers 36 include two sheets of planar doping of highly dopedp-material separated by a lightly doped layer of p-material. Thesep-type layers are offset from the n-type modulation doped quantum wellstructure 32 by the undoped spacer material 34. In this configuration,the top charge sheet achieves low gate contact resistance and the bottomcharge sheet defines the capacitance of the n-channel HFET with respectto the n-type modulation doped QW structure 32. Deposited on p-typelayer(s) 36 is one or more p-type ohmic contact layer(s) 38, whichenables the formation of ohmic contacts thereto.

For the n-channel HFET device, a gate terminal electrode (two shown as51) of the n-channel HFET device is operably coupled to the top p-typeohmic contact layer(s) 38. A source terminal electrode 53 and a drainterminal electrode 55 of the re-channel HFET device are operably coupledto opposite ends of a QW channel(s) realized in the n-type modulationdoped QW structure 32. One or more terminal electrodes (such aselectrodes 59 and 61) can be operably coupled to the p-type modulationdoped QW structure 20 and used as collector terminal electrodes for then-channel HFET device.

For the p-channel HFET device, a gate terminal electrode (two shown as57) of the p-channel HFET device is operably coupled to the bottomn-type ohmic contact layer(s) 14. A source terminal electrode 59 and adrain terminal electrode 61 of the p-channel HFET device are operablycoupled to opposite ends of a QW channel(s) realized in the p-typemodulation doped QW structure 20. One or more terminal electrodes (suchas the electrodes 53 and 55) can be operably coupled to the n-typemodulation doped QW structure 32 and used as a collector terminalelectrode for the p-channel HFET device.

Both the n-channel HFET device and the p-channel HFET device are fieldeffect transistors where current flows as a two-dimensional gas througha QW channel with contacts at either end. The basic transistor action isthe modulation of the QW channel conductance by a modulated electricfield that is perpendicular to the QW channel. The modulated electricfield modulates the QW channel conductance by controlling an inversionlayer (i.e., a two-dimensional electron gas for the n-channel HFETdevice or a two-dimensional hole gas for the p-channel HFET) as afunction of gate voltage relative to source voltage.

For the n-channel HFET device, the QW channel conductance is turned onby biasing the gate terminal electrode 51 and source terminal electrode53 at voltages where the P/N junction of the gate and source regions isforward biased with minimal gate conduction and an inversion layer ofelectron gas is created in the QW channel of the n-type modulation dopedquantum well structure 32 between the source terminal electrode 53 andthe drain terminal electrode 55. In this configuration, the sourceterminal electrode 53 is the terminal electrode from which the electroncarriers enter the QW channel of the n-type modulation doped quantumwell structure 32, the drain terminal electrode 55 is the terminalelectrode where the electron carriers leave the device, and the gateterminal electrode 51 is the control terminal for the device.

FIG. 2 shows the current-voltage characteristics of an exemplaryn-channel HFET device. The n-channel HFET device can configured tooperate as a field effect electrical transistor by biasing the device,for example, as shown in the load bias line of FIG. 2. With V_(GS) setsuch that there is minimal gate conduction, an inversion layer ofelectrons (i.e., two-dimensional electron gas) is created in the QWchannel of the n-type modulation doped quantum well structure 32 and theapplication of a voltage V_(DS) causes current I_(D) to flow from thedrain terminal electrode 55 to the source terminal electrode 53. IfV_(GS) is set below the threshold voltage, the inversion layer ofelectrons (i.e., two-dimensional electron gas) is not formed in the QWchannel of the n-type modulation doped quantum well structure 32. Thechannel has in effect disappeared, and no current I_(D) flows from thedrain terminal electrode 55 to the source terminal electrode 53 (i.e.,I_(D)=0). Since the gate-channel junction is always biased such thatthere is minimal gate conduction of electrons, only a leakage currentflows through the gate terminal electrode 51. The collector terminal(s)electrode 59, 61 of the n-channel HFET device can be coupled to thesource terminal electrode 53 or possibly reverse biased with respect tothe source terminal electrode 53 and the drain terminal electrode 55 inorder to minimize such leakage current.

The p-channel HFET device operates in a similar manner to the n-channelHFET device with the current direction and voltage polarities reversedwith respect to those of the n-channel HFET device. For the p-channelHFET device, the QW channel conductance is turned on by biasing the gateterminal electrode 57 and the source terminal electrode 59 at a voltageswhere the P/N junction of the source and gate regions is forward-biasedwith minimal gate conduction and an inversion layer of hole gas iscreated in the QW channel of the p-type modulation doped quantum wellstructure 20 between the source terminal electrode 59 and the drainterminal electrode 61. In this configuration, the source terminalelectrode 59 is the terminal from which the hole carriers enter the QWchannel of the p-type modulation doped quantum well structure 20, thedrain terminal electrode 61 is the terminal where the hole carriersleave the device, and the gate terminal electrode 57 is the controlterminal for the device.

The device structure of the present application can also be configuredto realize bipolar inversion channel field-effect transistors (BICFET)with either an n-type modulation doped quantum well inversion channelbase region (n-channel base BICFET) or a p-type modulation doped quantumwell inversion channel base region (p-channel base BICFET) as shown inFIG. 3.

For the n-channel base BICFET device, an emitter terminal electrode 71of the n-channel base BICFET device is operably coupled to the topp-type ohmic contact layer(s) 38 of the active device structure. A baseterminal electrode 73 of the n-channel base BICFET device is operablycoupled to the QW channel(s) realized in the n-type modulation doped QWstructure 32. A collector terminal electrode 75 of the n-channel baseBICFET device is operably coupled to the p-type modulation doped QWstructure 20. The n-channel base BICFET device is a bipolar junctiontype transistor which can be operated in an active mode by applying aforward bias to the PN junction of the emitter and base regions whileapplying a reverse bias to the PN junction of the base and collectorregions, which causes holes to be injected from the emitter terminalelectrode 71 to the collector terminal electrode 75. Because the holesare positive carriers, their injection contributes to current flowingout of the collector terminal electrode 75 as well as current flowinginto the emitter terminal electrode 71. The bias conditions also causeelectrons to be injected from the base to the emitter, which contributesto current flowing out of the base terminal electrode 73 as well as thecurrent flowing into the emitter terminal electrode 71.

For the p-channel base BICFET device, an emitter terminal electrode 77of the p-channel base BICFET device is operably coupled to the bottomn-type ohmic contact layer(s) 14 of the active device structure. A baseterminal electrode 75 of the p-channel base BICFET device is operablycoupled to the QW channel(s) realized in the p-type modulation doped QWstructure 20. A collector terminal electrode 73 of the p-channel baseBICFET device is operably coupled to the n-type modulation doped QWstructure 32. The p-channel base BICFET device is a bipolar junctiontype transistor which can be operated in an active mode by applying aforward bias to the PN junction of the emitter and base regions whileapplying a reverse bias to the PN junction of the base and collectorregions, which causes electrons to be injected from the emitter terminalelectrode 77 to the collector terminal electrode 73. Because theelectrons are negative carriers, their injection contributes to currentflowing into the collector terminal electrode 73 as well as currentflowing out of the emitter terminal electrode 77. The bias conditionsalso cause holes to be injected from the base to the emitter, whichcontributes to current flowing into the base terminal electrode 75 aswell as the current flowing out of the emitter terminal electrode 77.

FIG. 4 shows the current-voltage characteristics of an exemplaryn-channel base BICFET device. The n-channel base BICFET device canconfigured to operate as a bipolar junction transistor in its activemode by biasing the device, for example, as shown in the load bias lineof FIG. 4 where a forward bias is applied to the PN junction of theemitter and base regions while a reverse bias is applied to the PNjunction of the base and collector. In this active mode configuration,the collector current I_(C) flowing out of the collector can be relatedto the base current I_(B) flowing out of the base by a simple idealequation I_(C)=β_(f)*I_(B) where the parameter β_(f) is the forward DCcurrent gain of the transistor. The simple ideal equation can beadjusted to account for the non-zero slope of voltage-currentcharacteristics for the constant current region of operation utilizingan early voltage parameter (such as V_(A) or r₀) as is well known. Thep-channel base BICFET operates in a similar manner to the n-channel HFETdevice with the current directions and voltage polarities reversed withrespect to those of the n-channel base BICFET device.

The device structure of the present application can also be configuredto realize optoelectronic devices such as an electrically-pumped laseror optical detector as shown in FIG. 5. To form a resonant cavity devicefor optical signal emission and/or detection, a top DBR mirror 40 can beformed over the active device structure described above. The top DBRmirror can be formed by depositing pairs of semiconductor or dielectricmaterials with different refractive indices. The distance between thetop DBR mirror 40 and bottom DBR mirror 12 represents the length of theoptical cavity and can be set to correspond to the designated wavelength(such as 1 to 3 times the designated wavelength). This distance can takeinto account the penetration depth of the light into the bottom and topDBR mirror. This distance is controlled by adjusting the thickness ofone or more of the layers therebetween to enable this condition. Forconfigurations where light is input into and/or emitted from the devicelaterally (i.e., from a direction normal to the cross section of FIG.5), a diffraction grating can be formed in the top DBR mirror 40 overthe active device structure described above. When the device isoperating in the lasing mode, the diffraction grating performs thefunction of diffracting light produced by the vertical cavity into lightpropagating laterally in a waveguide which has the top DBR mirror 40 andbottom DBR mirror 12 as waveguide cladding layers and which has lateralconfinement regions (typically formed by implants as described herein inmore detail). When the device is operating in the optical detectionmode, the diffraction grating performs the function of diffractingincident light that is propagating in the lateral direction into thevertical cavity mode, where it is absorbed resonantly in the verticalcavity. Alternatively, light may enter and exit the resonant verticalcavity through an optical aperture (not shown) in the top surface of thedevice. In this case, the diffraction grating is omitted, the top DBRmirror 40 defines a cavity for the vertical emission and absorption oflight, and the device operates as a vertical cavity surface emittinglaser/detector.

For the laser or detector of FIG. 5, an anode terminal electrode 81 ofthe quantum well laser or detector can be operably coupled to the topp-type ohmic contact layer(s) 38. A cathode terminal electrode 83 of thequantum well laser or detector can be operably coupled to the n-typemodulation doped QW structure 32. One or more optional electrodes can beoperably coupled to the p-type modulation doped QW structure 20 as wellas to the bottom n-type ohmic contact layer(s) 14. If present, theseoptional electrodes are configured to float electrically with respect tothe electrical signals of the anode terminal electrode 81 as well as ofthe cathode terminal electrode 83. In this manner, the p-type region ofthe p-type modulation doped QW structure 20 floats with respect to theelectrical signals of the anode terminal electrode 81 as well as of thecathode terminal electrode 83.

The device of FIG. 5 can be configured to operate as anelectrically-pumped laser emitter by forward biasing the anode terminalelectrode 81 relative to the cathode terminal electrode 83 such thatholes are injected from the anode terminal electrode 81 into the QWchannel(s) realized in the n-type modulation doped QW structure 32. Thelower p-type region of the active device structure (which includes thep-type modulation doped QW structure 20) floats with respect to theelectrical signals of the anode terminal electrode 81 as well as of thecathode terminal electrode 83. As shown in the energy band diagram ofFIG. 6, hole thermionic current flows into this p-type region and thepositive hole carriers build up there and cause this p-type region tobecome self-biased, i.e. it moves downwards in FIG. 6. This has theeffect of reducing the energy difference between the QD-in-QW structure28 and this p-type region, which enables hole carriers to diffuse towardthe QD-in-QW structure 28 and populate the QDs embedded in the QD-in-QWstructure 28. At the same time, the self-biasing of the p-type regionalso reduces the barrier for electrons in the QW channel of the n-typemodulation doped QW structure 32 to flow toward the QD-in-QW structure28 by thermal excitation in the conduction band and populate the QDsembedded in the QD-in-QW structure 28. The simultaneous injection ofholes and electrons into the QDs embedded in the QD-in-QW structure 28allows for laser emission arising from spontaneous emission andstimulated emission of photons in the QDs embedded in the QD-in-QWstructure 28. The confinement of carriers afforded by the QDs of theQD-in-QW structure 28 enhances the interaction between the carriers andradiation that results in the laser emission.

FIG. 7 shows the current-voltage characteristics of an exemplary quantumwell laser device. The quantum well laser can configured to emit lightby biasing the device, for example, as shown in the load bias line ofFIG. 7 where the anode terminal electrode 81 is forward biased withrespect to the cathode terminal electrode 83 in order to produce acurrent I through the device that is larger than the threshold lasingcurrent I_(TH) as shown. The quantum well laser can configured into anoff state that does not emit light by biasing the device such thatcurrent I through the device that is less than the threshold lasingcurrent I_(TH) (for example, in the cutoff region where the current I isat or near 0).

The device of FIG. 5 can be configured to operate as an optical detectorby reverse biasing the anode terminal electrode 81 relative to thecathode terminal electrode 83. The lower p-type region of the activedevice structure (which includes the p-type modulation doped QWstructure 20) floats with respect to the electrical signals of the anodeterminal electrode 81 as well as of the cathode terminal electrode 83.The reverse bias conditions are selected such that the device producesphotocurrent proportional to the intensity of an optical input signalabsorbed by the QDs embedded in the QD-in-QW structure 28. Thephotocurrent is derived from optical absorption in the QDs wherebyphotons collide with a valence electron and elevate the electron intothe conduction band. This interaction creates an electron in theconduction band and a hole in the valence band—an electron-hole pair.The electron-hole pair contributes to the photocurrent generated by thedevice in response to the optical input signal. The confinement ofcarriers afforded by the QDs of the QD-in-QW structure 28 enhances theinteraction between the carriers and the incident photons.

The device structure of the present application can also be configuredto realize an electrically-pumped laser or optical detector as shown inFIG. 8. To form a resonant cavity device for optical signal emissionand/or detection, a top DBR mirror 40 is formed over the active devicestructure as described above. For configurations where light is inputinto and/or emitted from the device laterally (i.e., from a directionnormal to the cross section of FIG. 8), diffraction grating can beformed in the top DBR mirror 40 over the active device structure asdescribed above. Alternatively, light may enter and exit the resonantvertical cavity through an optical aperture (not shown) in the topsurface of the device. In this case, the diffraction grating is omitted,the top DBR mirror 40 defines a cavity for the vertical emission andabsorption of light, and the device operates as a vertical cavitysurface emitting laser/detector.

For the electrically-pumped laser or detector of FIG. 8, an anodeterminal electrode 91 of the laser or detector is operably coupled tothe p-type modulation doped QW structure 20. A cathode terminalelectrode 93 of the laser or detector is operably coupled to the bottomn-type ohmic contact layer(s) 14. One or more optional electrodes can beoperably coupled to the n-type modulation doped QW structure 32 as wellas to the top p-type ohmic contact layer(s) 38. If present, theseoptional electrodes are configured to float electrically with respect tothe electrical signals of the anode terminal electrode 91 as well as ofthe cathode terminal electrode 93. In this manner, the n-type region ofthe n-type modulation doped QW structure 32 floats with respect to theelectrical signals of the anode terminal electrode 91 as well as of thecathode terminal electrode 93.

The device of FIG. 8 can be configured to operate as anelectrically-pumped laser by forward biasing the anode terminalelectrode 91 relative to the cathode terminal electrode 93 such thatholes are injected from the anode terminal electrode 91 into the QWchannel(s) realized in the p-type modulation doped QW structure 20. Then-type region of the n-type modulation doped QW structure 32 floats withrespect to the electrical signals of the anode terminal electrode 81 aswell as of the cathode terminal electrode 83. Similar to the operationof the optical devices of FIG. 5 with the carriers reversed, electronthermionic current flows into this n-type region and the negativeelectron carriers build up there and cause this n-type region to becomeself-biased. This has the effect of reducing the energy differencebetween the QD-in-QW structure 24 and this n-type region, which enableselectron carriers to diffuse toward the QD-in-QW structure 24 andpopulate the QDs embedded in the QD-in-QW structure 24. At the sametime, the self-biasing of the n-type region also reduces the barrier forholes in the QW channel of the p-type modulation doped QW structure 20to flow toward the QD-in-QW structure 24 by thermal excitation in thevalence band and populate the QDs embedded in the QD-in-QW structure 24.The simultaneous injection of holes and electrons into the QDs embeddedin the QD-in-QW structure 24 allows for laser emission arising fromspontaneous emission and stimulated emission of photons in the QDsembedded in the QD-in-QW structure 24. The confinement of carriersafforded by the QDs of the QD-in-QW structure 24 enhances theinteraction between the carriers and radiation that results in the laseremission. The current-voltage characteristics of the quantum well laserdevice can be similar to that shown in FIG. 7.

The device of FIG. 8 can also be configured to operate as an opticaldetector by reverse biasing the anode terminal electrode 91 relative tothe cathode terminal electrode 93. The n-type region of the modulationdoped QW structure 32 floats with respect to the electrical signals ofthe anode terminal electrode 91 as well as of the cathode terminalelectrode 93. The reverse bias conditions are selected such that thedevice produces photocurrent proportional to the intensity of an opticalinput signal absorbed by the QDs embedded in the QD-in-QW structure 24.The photocurrent is derived from optical absorption in the QDs wherebyphotons collide with a valence electron and elevate the electron intothe conduction band. This interaction creates an electron in theconduction band and a hole in the valence band—an electron-hole pair.The electron-hole pair contributes to the photocurrent generated by thedevice in response to the optical input signal. The confinement ofcarriers afforded by the QDs of the QD-in-QW structure 24 enhances theinteraction between the carriers and the incident photons.

The device structure of the present application can also be configuredto realize an electrically-pumped thyristor laser or optical detector asshown in FIG. 9. To form a resonant cavity device for optical signalemission and/or detection, a top DBR mirror 40 is formed over the activedevice structure as described above. For configurations where light isinput into and/or emitted from the device laterally (i.e., from adirection normal to the cross section of FIG. 9), a diffraction gratingcan be formed in the top DBR mirror 40 over the active device structureas described above. Alternatively, light may enter and exit the resonantvertical cavity through an optical aperture (not shown) in the topsurface of the device. In this case, the diffraction grating is omitted,the top DBR mirror 40 defines a cavity for the vertical emission andabsorption of light, and the device operates as a vertical cavitysurface emitting laser/detector.

For the thyristor laser or detector device of FIG. 9, an anode terminalelectrode (two parts shown as 101) is operably coupled to the top p-typecontact layer 38. A bottom cathode terminal electrode (two parts shownas 107) is operably coupled to the bottom n-type contact layer 14. Ann-channel injector terminal (two parts shown as 103) is operably coupledto the n-type modulation doped QW structure 32. A p-channel injectorterminal (two parts shown as 105) is operably coupled to the p-typemodulation doped QW structure 20.

For the electrically-pumped thyristor laser, the device switches from anon-conducting/OFF state (where the current I through the device issubstantially zero) to a conducting/ON state (where current I issubstantially greater than zero) when i) the anode terminal electrode101 is forward biased with respect to the cathode terminal electrode 107and ii) the voltage between n-channel injector 103 and the anodeelectrode 101 is biased such that charge is produced in the n-typemodulation doped QW structure 32 that is greater than the criticalswitching charge Q_(CR), which is that charge that reduces the forwardbreakdown voltage such that no off state bias point exists. The voltagebetween p-channel injector electrode 105 and cathode electrode 107 canalso be configured to produce a charge in the p-type modulation doped QWstructure 20 that is greater than the critical switching charge Q_(CR).The critical switching charge Q_(CR) is unique to the geometries anddoping levels of the device. The device switches from the conducting/ONstate (where the current I is substantially greater than zero) to anon-conducting/OFF state (where current I is substantially zero) whenthe current I through device falls below the hold current of the devicefor a sufficient period of time such that the charge in the n-typemodulation doped QW structure 32 (or the charge in the p-type modulationdoped QW structure 20) decreases below the holding charge Q_(H), whichis the critical value of the channel charge which will sustain holdingaction. Thus, if the anode terminal 101 is forward biased with respectto the cathode terminal 107 and the n-channel injector 103 (and/or thep-channel injector terminal 105) is biased to produce the criticalswitching charge Q_(CR) in the n-type modulation doped QW structure 32(or the p-type modulation doped QW structure 20), then the thyristordevice will switch to its conducting/ON state. If the current I in theconducting/ON state is above the threshold for lasing I_(TH), then laseremission will occur. In this configuration, the current I involves thesimultaneous injection of holes and electrons into the QDs embedded inthe QD-in-QW structures 24 and 28 to allow for laser emission arisingfrom spontaneous emission and stimulated emission of photons in the QDsembedded in the QD-in-QW structures 24 and 28. The confinement ofcarriers afforded by the QDs of the QD-in-QW structures 24 and 28enhances the interaction between the carriers and radiation that resultsin the laser emission.

FIG. 10 shows the current-voltage characteristics of an exemplarythyristor laser device under a variety of injector bias conditions wherean injector current I_(g) supplies charge (electrons) to the n-typemodulation-doped QW structure of the device. The thyristor laser devicecan configured to emit light by biasing the device, for example, asshown in the load bias line of FIG. 10 where the anode terminalelectrode 101 is forward biased with respect to the cathode electrode107 in order to produce a current I through the device that is largerthan the threshold lasing current I_(TH) as shown. The thyristor laserdevice can configured into an off state that does not emit light bybiasing the device such that current I through the device that is lessthan the threshold lasing current I_(TH) (for example, in the cutoffregion where the current I is at or near 0).

For the thyristor detector, the device switches from anon-conducting/OFF state (where the current I through the device issubstantially zero) to a conducting/ON state (where current I issubstantially greater than zero) in response to an input optical pulsethat produces charge in the n-type modulation doped QW structure 32and/or the p-type modulation doped QW structure 20 resulting from photonabsorption of the input optical pulse. Specifically, the anode terminalelectrode 101 is forward biased with respect to the cathode terminalelectrode 107 and the voltage between n-channel injector 103 and theanode electrode 101 (and/or the voltage between the p-channel injector105 and the cathode terminal electrode 107) is biased such that thatcharged produced in the n-type modulation doped QW structure 32 (and/orthe p-type modulation doped QW structure 20) resulting from photonabsorption of the input optical pulse is greater than the criticalswitching charge Q_(CR). When the input optical pulse is removed, thedevice switches from the conducting/ON state (where the current I issubstantially greater than zero) to a non-conducting/OFF state (wherecurrent I is substantially zero) when the charge in the n-typemodulation doped QW structure 32 (and/or the p-type modulation doped QWstructure 20) decreases below the holding charge Q_(H). The confinementof carriers afforded by the QDs of the QD-in-QW structures 24 and 28enhances the interaction between the carriers and the incident photons.

The self-assembled quantum dots (QDs) embedded within the QD-in-QWstructures 24 and 28 improves the efficiency of the optoelectronicdevices described herein. Specifically, the population inversionnecessary for laser action and amplification and the photon absorptionmechanism for necessary for optical detection occurs more efficientlywith the introduction of the quantum dots and thus decreases thenecessary current required for lasing action and amplification increasesthe photocurrent produced by absorption. Furthermore, the size of theembedded QDs can be controlled to dictate the wavelength of the desiredoptical function (emission for lasing, amplification, absorption fordetection). For example, the size of the QDs in either or both QD-in-QWstructures 22, 26 can be controlled to dictate the wavelength in rangefrom 1300 nm up to 1550 nm for use in the O to L (1260-1625 nm) bandsemployed in commercial optical telecommunication networks. Furthermore,the density distribution of the embedded QDs can be controlled todictate the laser output power. High density of embedded QDs can providefor an increase of laser output power, but require a greater thresholdlasing current.

In one embodiment, the QD-in-QW structure 24 is realized by first andsecond bilayer structures with an undoped barrier layer therebetween.Both the first and second bilayer structures include a templatesubstructure offset from an emission substructure by a thin undopedbarrier layer as shown in FIG. 11A. More specifically, the QD-in-QWstructure 24 includes the first bilayer structure including a templatesubstructure 24 a, a thin undoped barrier layer 24 b, and an emissionsubstructure 24 c. An undoped barrier layer 24 d is deposited on theemission substructure 24 c. The second bilayer structure is formed onthe barrier layer 24 d and includes a template substructure 24 e, a thinundoped barrier layer 24 f, and an emission substructure 24 g as shownin FIG. 11A.

The template substructure 24 a is formed on the undoped spacer layer 22that lies above the p-type modulation doped QW structure 20. The spacerlayer 22 acts as a barrier between the QDs embedded in the templatesubstructure 24 a and the QW(s) of the underlying p-type modulationdoped QW structure 20. The spacer layer 22 is realized from a materialwith a higher bandgap energy than the QW material of the templatesubstructure 24 a (such as a GaAs spacer layer to support an InGaAs QWmaterial of the template structure 24 a). Similarly, the templatesubstructure 24 e is formed on the undoped barrier layer 24 d. Thetemplate substructures 24 a and 24 e each include a digitally-graded QWwith self-assembled QDs embedded therein. The self-assembled QDs can beformed during molecular beam epitaxy growth by a self-assembly methodknown as the Stranski-Krastanov process. In this process, an initiallayer (such as InGaAs) that is part a digitally-graded quantum well(such as a digitally graded InGaAs QW) is deposited. A compoundsemiconductor that is lattice mismatched relative to the initial layerand underlying layer is deposited on the initial layer (such as latticemismatched InAs grown in an InGaAs QW initial layer above GaAs). Inparticular, the lattice mismatch of the compound semiconductor is suchthat the growth forms three dimensional islands after a deposition of acritical thickness of the compound semiconductor. The growth iscontinued to allow the three dimensional islands to expand to form theself-assembled QDs that have the desired characteristic dimensionalrange. After the self-assembled QDs are formed on the initial layer, thedeposition of the digitally-graded QW material resumes such that theself-assembled QDs are covered and fully embedded within thedigitally-graded QW material.

The thin undoped barrier layer 24 b is deposited on the templatesubstructure 24 a. Similarly, the thin undoped barrier layer 24 f isdeposited on the template substructure 24 e. The undoped barrier layers24 b and 24 f are each realized from a material with a higher bandgapenergy than the QW material of both the template substructure and theemission substructure (such as a GaAs barrier layer to support an InGaAsQW material of the template substructure and the emission substructure).

The emission substructure 24 c is formed on the barrier layer 24 b.Similarly, the emission substructure 24 g is formed on the barrier layer24 f. The emission substructures 24 c and 24 g each include ananalog-graded QW with self-assembled QDs embedded therein. Theself-assembled QDs can be formed during molecular beam epitaxy growth bya self-assembly method known as the Stranski-Krastanov process similarto the growth conditions of the template substructure. In this process,an initial layer (such as InGaAs) that is part an analog-graded quantumwell (such as an analog-graded InGaAs QW) is deposited. A compoundsemiconductor that is lattice mismatched relative to the initial layerand underlying layer is deposited on the initial layer (such as latticemismatched InAs grown in an InGaAs QW initial layer above GaAs). Inparticular, the lattice mismatch of the compound semiconductor is suchthat the growth forms three dimensional islands after a deposition of acritical thickness of the compound semiconductor. The three dimensionalislands of the emission substructure are formed such that they arealigned with the self-assembled QDs of the underlying templatestructure. The growth is continued to allow the three dimensionalislands to expand to form the self-assembled QDs that have the desiredcharacteristic dimensional range. After the self-assembled QDs areformed on the initial layer, the deposition of the analog-graded QWmaterial resumes such that the self-assembled QDs are covered and fullyembedded within the analog-graded QW of the respective emissionstructure.

The undoped barrier layer 24 d is formed between the emissionsubstructure 24 c and the template substructure 24 e of the respectivebilayer structures. The barrier layer 24 d is realized from a materialwith a higher bandgap energy than the QW material of both the templatesubstructure 24 e and the emission substructure 24 c (such as a GaAsbarrier layer to support an InGaAs QW material of the templatesubstructure 24 e and the emission substructure 24 c).

The spacer layer(s) 26 is formed on the emission substructure 24 g asshown.

The QD-in-QW structure 28 can also be realized by bilayer structuresthat include first and second bilayer structures as shown in FIG. 11B.Both the first and second bilayer structures include a templatesubstructure offset from an emission substructure by a thin undopedbarrier layer as shown in FIG. 11B. More specifically, the QD-in-QWstructure 28 includes an undoped spacer layer 28 a followed by the firstbilayer structure including a template substructure 28 b, a thin undopedbarrier layer 28 c, and an emission substructure 28 d. An undopedbarrier layer 28 e is deposited on the emission substructure 28 d. Thesecond bilayer structure is formed on the barrier layer 28 e andincludes a template substructure 28 f, a thin undoped barrier layer 28g, and an emission substructure 28 h as shown in FIG. 11B.

The template substructure 28 b is formed on the undoped barrier layer 28a that lies above the spacer layer(s) 26. The barrier layer 28 a isrealized from a material with a higher bandgap energy than the QWmaterial of the template substructure 28 b (such as a GaAs spacer layerto support an InGaAs QW material of the template substructure 28 b).Similarly, the template substructure 28 f is formed on the undopedbarrier layer 28 e. The template substructures 28 b and 28 f eachinclude a digitally-graded QW with self-assembled QDs embedded therein.The self-assembled QDs can be formed during molecular beam epitaxygrowth by a self-assembly method known as the Stranski-Krastanovprocess. In this process, an initial layer (such as InGaAs) that is parta digitally-graded quantum well (such as a digitally graded InGaAs QW)is deposited. A compound semiconductor that is lattice mismatchedrelative to the initial layer and underlying layer is deposited on theinitial layer (such as lattice mismatched InAs grown in an InGaAs QWinitial layer above GaAs). In particular, the lattice mismatch of thecompound semiconductor is such that the growth forms three dimensionalislands after a deposition of a critical thickness of the compoundsemiconductor. The growth is continued to allow the three dimensionalislands to expand to form the self-assembled QDs that have the desiredcharacteristic dimensional range. After the self-assembled QDs areformed on the initial layer, the deposition of the digitally-graded QWmaterial resumes such that the self-assembled QDs are covered and fullyembedded within the digitally-graded QW material.

The thin undoped barrier layer 28 c is deposited on the templatesubstructure 28 b. Similarly, the thin undoped barrier layer 28 g isdeposited on the template substructure 28 f. The undoped barrier layers28 c and 28 g are each realized from a material with a higher bandgapenergy than the QW material of both the underlying template substructureand the overlying emission substructure (such as a GaAs barrier layer tosupport an InGaAs QW material of the underlying template substructureand the overlying emission substructure).

The emission substructure 28 d is formed on the barrier layer 28 c.Similarly, the emission substructure 28 h is formed on the barrier layer28 g. The emission substructures 28 d and 28 c each include ananalog-graded QW with self-assembled QDs embedded therein. Theself-assembled QDs can be formed during molecular beam epitaxy growth bya self-assembly method known as the Stranski-Krastanov process similarto the growth conditions of the template substructure. In this process,an initial layer (such as InGaAs) that is part an analog-graded quantumwell (such as an analog-graded InGaAs QW) is deposited. A compoundsemiconductor that is lattice mismatched relative to the initial layerand underlying layer is deposited on the initial layer (such as latticemismatched InAs grown in an InGaAs QW initial layer above GaAs). Inparticular, the lattice mismatch of the compound semiconductor is suchthat the growth forms three dimensional islands after a deposition of acritical thickness of the compound semiconductor. The three dimensionalislands of the emission substructure are formed such that they arealigned with the self-assembled QDs of the underlying templatesubstructure. The growth is continued to allow the three dimensionalislands to expand to form the self-assembled QDs that have their desiredcharacteristic dimensional range. After the self-assembled QDs areformed on the initial layer, the deposition of the analog-graded QWmaterial resumes such that the self-assembled QDs are covered and fullyembedded within the analog-graded QW of the respective emissionsubstructure.

The undoped spacer layer(s) 30 is formed on the emission substructure 28h as shown. The undoped spacer layer 20 acts as a barrier between theQDs embedded in the emission substructure 28 h and the QW(s) of theoverlying n-type modulation doped QW structure 32.

The size of the QDs of the template and emission substructures of theQD-in-QW structures 24 and 28 dictates the wavelength of theelectromagnetic radiation emitted or absorbed for the desired opticalfunction (laser emission, amplification, optical detection). Forexample, the size of the QDs in the QD-in-QW structures 24 and 28 can becontrolled to dictate the emission/absorption wavelength in range from1300 nm up to 1550 nm for use in the O to L (1260-1625 nm) bandsemployed in commercial optical telecommunication networks. Furthermore,the characteristic emission/absorption wavelengths can be different forthe QD-in-QW structures 24 and 28. For example, the size of the QDs inQD-in-QW structure 24 can be controlled to dictate theemission/absorption wavelength in range near 1310 nm, and the size ofthe QDs in the QD-in-QW structure 28 can be controlled to dictate theemission/absorption wavelength in range near 1550 nm.

Furthermore, the density distribution of the QDs of the template andemission substructures dictates the laser output power. A high densityof embedded QDs can provide for an increase of laser output power, butrequire a greater threshold lasing current. The density distribution ofthe QDs of the template substructures dictates the density distributionof the QDs of the adjacent emission substructure and allows the growthconditions of the emission substructure to be tuned to control the sizeof the QDs of the adjacent emission substructure. Furthermore, thetemplate substructure relaxes the strain mismatch of the emissionsubstructure that arises from the layer underlying the templatesubstructure and thus allows for the larger sized QDs to be assembled inthe adjacent emission substructure.

FIGS. 12A-C illustrates an exemplary layer structure utilizing groupIII-V materials for realizing the multilayer structures describedherein. The layer structure of FIGS. 12A-C can be made, for example,using known molecular beam epitaxy (MBE) techniques. As shown, asemiconductor layer 1003 of AlAs and a semiconductor layer 1005 of GaAsare alternately deposited (with preferably at least seven pairs) upon asemi-insulating gallium arsenide substrate 1001 in sequence to form thebottom distributed bragg reflector (DBR) mirror. The number of AlAslayers will preferably always be one greater than the number of GaAslayers so that the first and last layers of the mirror are shown aslayer 1003. In the preferred embodiment, the AlAs layers 1003 aresubjected to high temperature steam oxidation during fabrication toproduce the compound Al_(x)O_(y) so that a mirror will be formed at thedesigned center wavelength. This center wavelength is selected such thatall of the resonant wavelengths for the various cavities of the arraywill be subject to high reflectivity. Therefore the thicknesses oflayers 1003 and 1005 in the mirror are chosen so that the final opticalthickness of GaAs and Al_(x)O_(y) are one quarter wavelength of thecenter wavelength λ_(D). Alternatively the mirrors could be grown asalternating layers of one-quarter wavelength thickness of GaAs and AlAsat the designed wavelength so that the oxidation step is not used. Inthat case, many more pairs are required (with typical numbers such as 22pairs) to achieve the reflectivity needed for efficient optical lasingand detection.

Deposited upon the last bottom mirror layer 1003 is the active devicestructure which begins with layer 1009 of N+ type GaAs that enables theformation of ohmic contacts thereto (for example, when contacting to thegate terminal of an inverted p-channel HFET device, the emitter terminalof the p-channel base BICFET device, the cathode terminal of the quantumwell laser/detector device, and the cathode terminal of the thyristorlaser or detector device). Layer 1009 has a typical thickness of 3000 Åand a typical n-type doping of 3.5×10¹⁸ cm⁻³. The N+ doped GaAs layer1009 corresponds to the ohmic contact layer 14 of FIGS. 1, 3, 5, 8, 9 asdescribed above.

Deposited on layer 1009 is layer 1011 of n-type Al_(x1)Ga_((1-x1))Aswith a typical thickness of 600-1000 Å and a typical doping of 1×10¹⁸cm⁻³. The parameter x1 is preferably in the range between 70% and 80%for layer 1009. This layer serves as part of the gate region of thep-channel HFET device and optically as a small part of the lowerwaveguide cladding of the respective optical device. Note that amajority of the lower waveguide cladding for waves propagating in theguide formed by the optically active region of the device is provided bythe lower DBR mirror itself. The lower DBR mirror causes the light to beguided partially as a dielectric waveguide and partially as a mirrorwaveguide. Next are four layers (1013, 1014, 1015, 1016) ofAl_(x2)Ga_((1-x2))As. These four layers collectively have a totalthickness of about 230-430 Å and where x2 is about 15%. The first layer1013 is about 60 Å thick and is doped N+ type in the form of deltadoping with a typical n-type doping of 3.5×10¹⁸ cm⁻³. The second layer1014 is about 100-300 Å thick and is undoped. The third layer 1015 isabout 40 Å thick and is doped P+ type in the form of delta doping with atypical p-type doping of 7×10¹⁸ cm⁻³. The fourth layer 1016 is about 30Å thick and is undoped to form a spacer layer. This layer forms thelower separate confinement heterostructure (SCH) layer for the opticaldevices. The n-type AlGaAs layers 1011 and 1013 correspond to the n-typelayer(s) 16 of FIGS. 1, 3, 5, 8, and 9 as described above. The undopedAlGaAs layer 1014 corresponds to the spacer layer 18 of FIGS. 1, 3, 5,8, and 9 as described above.

Next is an undoped GaAs barrier layer 1017 and an InGaAs quantum welllayer 1019 repeated for a number of quantum wells (such as three or morequantum wells) for the inverted p-type modulation doped quantumstructure. Single quantum well structures may also be used. The undopedGaAs barrier layer 1017 preferably is about 10 Å thick. The InGaAsquantum well layer 1019 is preferably about 60 Å thick. Layers 1015 to1019 correspond to the inverted p-type modulation doped quantumstructure 20 of FIGS. 1, 3, 5, 8, and 9 as described above.

An undoped GaAs spacer layer 1021 that is about 300-500 Å thick followsthe last InGaAs quantum well layer 1019. Layer 1021 corresponds tospacer layer 22 of FIGS. 1, 3, 5, 8, and 9 as described above.

Following the spacer layer 22 are layers 1024 to 1051 that correspond tothe QD-in QW structure 24 of FIG. 11A for the devices of FIGS. 1, 3, 5,8, and 9 as described above. Layers 1023 to 1027 form the templatestructure 24 a of FIG. 11A with QDs embedded within an InGaAs quantumwell that employs digital grading of In with 15-20% In. The initiallayer 1023 of the InGaAs quantum well that is deposited before the QDgrowth sequence (specified as 1025) is preferably about 2 Å thick. Thelayer 1027 of the InGaAs quantum well that is deposited after the QDgrowth sequence is preferably about 40-60 Å thick. An undoped GaAsbarrier layer 1029 is deposited on the InGaAs quantum well layer 1027.The undoped GaAs barrier layer 1029 is preferably about 100 Å thick andcorresponds to the barrier layer 24 b of FIG. 11A.

Layers 1031 to 1035 form the emission structure 24 c of FIG. 11A withQDs embedded within an InGaAs quantum well that employs analog gradingof In. The initial layer 1031 of the InGaAs quantum well that isdeposited before the QD growth sequence (specified as 1033) ispreferably about 40 Å thick and has analog grading of In in the rangefrom In(0.375)Ga(0.9625)As at the interface to barrier layer 1029 toIn(0.3)Ga(0.7)As at the interface of the QD growth sequence. The layer1035 of the InGaAs quantum well that is deposited after the QD growthsequence (specified as 1033) is preferably about 40 Å thick and hasanalog grading of In in the range from In(0.3)Ga(0.7)As at the interfaceof the QD growth sequence to In(0.375)Ga(0.9625)As at the interface tobarrier layer 1037.

An undoped GaAs barrier layer 1037 is deposited on the InGaAs quantumwell layer 1035. The undoped GaAs barrier layer 1037 is preferably about300-500 Å thick and corresponds to the barrier layer 24 d of FIG. 11A.

Following barrier layer 1037 is layers 1039 to 1043 that form thetemplate structure 24 e of FIG. 11A with QDs embedded within an InGaAsquantum well that employs digital grading of In with 15-20% In. Theinitial layer 1039 of the InGaAs quantum well that is deposited beforethe QD growth sequence (specified as 1041) is preferably about 2 Åthick. The layer 1043 of the InGaAs quantum well that is deposited afterthe QD growth sequence is preferably about 40-60 Å thick. An undopedGaAs barrier layer 1045 is deposited on the InGaAs quantum well layer1043. The undoped GaAs barrier layer 1045 is preferably about 100 Åthick and corresponds to the barrier layer 24 f of FIG. 11A.

Following barrier layer 1045 is layers 1047 to 1051 that form theemission structure 24 g of FIG. 11A with QDs embedded within an InGaAsquantum well that employs analog grading of In. The initial layer 1047of the InGaAs quantum well that is deposited before the QD growthsequence (specified as 1049) is preferably about 40 Å thick and hasanalog grading of In in the range from In(0.375)Ga(0.9625)As at theinterface to barrier layer 1045 to In(0.3)Ga(0.7)As at the interface ofthe QD growth sequence. The layer 1051 of the InGaAs quantum well thatis deposited after the QD growth sequence is preferably about 40 Å thickand has analog grading of In in the range from In(0.3)Ga(0.7)As at theinterface of the QD growth sequence to In(0.375)Ga(0.9625)As at theinterface to next layer 1053.

Next are two layers (1053, 1055) of Al₂Ga_((1-x2))As. These two layerscollectively have a total thickness of about 4000 Å and where x2 isabout 15%. The first layer 1053 is about 2000 Å thick and is dopedP-type with a p-type doping of 5×10¹⁵ cm⁻³. The second layer 1055 isabout 2000 Å thick and is doped n-type with an n-type doping of 1-2×10¹⁶cm⁻³. Layers 1053 and 1055 correspond to the spacer layer(s) 26 of FIGS.1, 3, 5, 8, and 9 as described above.

Following the spacer layers 1053 and 1055 are layers 1057 to 1087 thatcorrespond to the QD-in QW structure 28 of FIG. 11B for the devices ofFIGS. 1, 3, 5, 8, and 9 as described above. Layer 1057 is an undopedGaAs barrier layer that is preferably on the order of 300-500 Å thickand corresponds to the barrier layer 28 a of FIG. 11B. Layers 1059 to1063 form the template structure 28 b of FIG. 11B with QDs embeddedwithin an InGaAs quantum well that employs digital grading of In with15-20% In. The initial layer 1059 of the InGaAs quantum well that isdeposited before the QD growth sequence (specified as 1061) ispreferably about 2 Å thick. The layer 1063 of the InGaAs quantum wellthat is deposited after the QD growth sequence is preferably about 40-60Å thick. An undoped GaAs barrier layer 1065 is deposited on the InGaAsquantum well layer 1063. The undoped GaAs barrier layer 1065 ispreferably about 100 Å thick and corresponds to the barrier layer 28 cof FIG. 11B.

Layers 1067 to 1071 form the emission structure 28 d of FIG. 11B withQDs embedded within an InGaAs quantum well that employs analog gradingof In. The initial layer 1067 of the InGaAs quantum well that isdeposited before the QD growth sequence (specified as 1069) ispreferably about 40 Å thick and has analog grading of In in the rangefrom In(0.375)Ga(0.9625)As at the interface to barrier layer 1065 toIn(0.3)Ga(0.7)As at the interface of the QD growth sequence. The layer1071 of the InGaAs quantum well that is deposited after the QD growthsequence is preferably about 40 Å thick and has analog grading of In inthe range from In(0.3)Ga(0.7)As at the interface of the QD growthsequence to In(0.375)Ga(0.9625)As at the interface to barrier layer1073.

An undoped GaAs barrier layer 1073 is deposited on the InGaAs quantumwell layer 1071. The undoped GaAs barrier layer 1073 is preferably about300-500 Å thick and corresponds to the barrier layer 28 e of FIG. 11B.

Following barrier layer 1073 is layers 1075 to 1079 that form thetemplate structure 28 f of FIG. 11B with QDs embedded within an InGaAsquantum well that employs digital grading of In with 15-20% In. Theinitial layer 1075 of the InGaAs quantum well that is deposited beforethe QD growth sequence (specified as 1077) is preferably about 2 Åthick. The layer 1079 of the InGaAs quantum well that is deposited afterthe QD growth sequence is preferably about 40-60 Å thick. An undopedGaAs barrier layer 1081 is deposited on the InGaAs quantum well layer1079. The undoped GaAs barrier layer 1081 is preferably about 100 Åthick and corresponds to the barrier layer 28 g of FIG. 11B.

Following barrier layer 1081 is layers 1083 to 1087 that form theemission structure 28 hg of FIG. 11B with QDs embedded within an InGaAsquantum well that employs analog grading of In. The initial layer 1083of the InGaAs quantum well that is deposited before the QD growthsequence (specified as 1085) is preferably about 40 Å thick and hasanalog grading of In in the range from In(0.375)Ga(0.9625)As at theinterface to barrier layer 1081 to In(0.3)Ga(0.7)As at the interface ofthe QD growth sequence. The layer 1087 of the InGaAs quantum well thatis deposited after the QD growth sequence is preferably about 40 Å thickand has analog grading of In in the range from In(0.3)Ga(0.7)As at theinterface of the QD growth sequence to In(0.375)Ga(0.9625)As at theinterface to the next layer 1089.

An undoped GaAs barrier layer 1089 is deposited on the InGaAs quantumwell layer 1087. The undoped GaAs barrier layer 1089 is preferably about300-500 Å thick and corresponds to the spacer layer 30 of FIG. 11B.

Next is an InGaAs quantum well layer 1091 an undoped GaAs barrier layer1093 that are repeated for a number of quantum wells (such as three ormore quantum wells) for the n-type modulation doped quantum structure.Single quantum well structures may also be used. The InGaAs quantum welllayer 1091 is preferably about 60 Å thick. The undoped GaAs barrierlayer 1093 is preferably about 10 Å thick.

Next are four layers (1095, 1097, 1099, 1101) of Al_(x2)Ga_((1-x2))As.These four layers collectively have a total thickness of about 270-470 Åand where x2 is about 15%. The first layer 1095 is about 30 Å thick andis undoped to form a spacer layer. The second layer 1097 is about 80 Åthick and is doped N+ type with an n-type doping of 3×10¹⁸ cm⁻³. Thethird layer 1099 is about 100-300 Å thick and is undoped. The fourthlayer 1101 is about 60 Å thick and is doped P+ type with a p-type dopingof 7×10¹⁸ cm⁻³. The layers 1097 to 1091 corresponds to the n-typemodulation doped quantum well structure 32 of FIGS. 1, 3, 5, 8, and 9 asdescribed above.

Next, a layer 1103 of p-type Al_(x1)Ga_((1-x1))As is deposited.Preferably, layer 1103 has a thickness on the order of 600-1000 Å andhas a P-type doping of 5×10¹⁷ cm⁻³. The parameter x1 of layer 1103 ispreferably about 70%. The undoped AlGaAs layer 1099 corresponds to theundoped spacer layer 34 of FIGS. 1, 3, 5, 8, and 9 as described above.Layers 1101 and 1103 corresponds to the p-type layer(s) 36 of FIGS. 1,3, 5, 8, and 9 as described above.

Deposited next are ohmic contact layers of GaAs (1105) and InGaAs(1107). Layer 1105 is about 500-1500 Å thick. Layer 1107 is about 30 Åthick. Both layers 1105 and 1107 are doped to a very high level of P+type doping (about 1×10²⁰ cm⁻³) to enable formation of ohmic contactsthereto. Layers 1105 and 1107 corresponds to the p-type ohmic contactlayer(s) 38 of FIGS. 1, 3, 5, 8, and 9 as described above.

The size of the embedded QDs of the template and emission substructuresof layers 1023-1051 and 1059-1087 contributes to the emission/absorptionwavelength of such structures. In one embodiment, the embedded QDs ofthe template and emission substructures of layers 1023-1051 and1059-1087 have the following characteristics:

-   -   QDs of the emission substructure having a maximal characteristic        dimension of 50-60 Å for production/absorption of light with a        characteristic wavelength at or near 1310 nm, and QDs of the        template substructure having a maximal characteristic dimension        of 20-30 Å (which are of smaller size that the emission        substructure) for production/absorption of light with a        characteristic wavelength at or near 1310 nm;    -   QDs of the emission substructure having maximal characteristic        dimension of 20-30 Å for production/absorption of light with a        characteristic wavelength at or near 1430 nm, and QDs of the        template substructure having a maximal characteristic dimension        of 20-30 Å (which are of smaller size that the emission        substructure) for production/absorption of light with a        characteristic wavelength at or near 1430 nm;    -   QDs of the emission substructure having a maximal characteristic        dimension of 100-110 Å for production/absorption of light with a        characteristic wavelength at or near 1550 nm, and QDs of the        template substructure having a maximal characteristic dimension        of 20-30 Å (which are of smaller size that the emission        substructure) for production/absorption of light with a        characteristic wavelength at or near 515 nm; and    -   QDs with an aspect ratio on the order of three (i.e., the        characteristic base dimension of the QD is about three times        larger than the characteristic height dimension of the QD).        Such QD size and aspect ratio are dictated by growth conditions,        particularly the number of monolayers for three dimensional InAs        QD growth. For example, 2 ML of three dimensional InAs QD growth        can be used for the template substructures, and 3.2 ML of three        dimensional InAs QD growth can be used for the emission        substructures. Other suitable monolayer growths can be used as        well. Moreover, the thickness of the barrier layer(s) between        the QDs of the template substructure and the emission        substructure can be controlled in order that the strain energy        from the template substructure have a desired influence on the        larger dot size and quality of the emission substructure. For        example, the barrier layers 1029, 1045, 1065, 1081 of FIGS.        12A-12C are 100 Å in thickness. Other suitable barrier        thicknesses can be used as well. Moreover, the In concentration        of the analog graded quantum well material onto which the QDs        are grown can be used to control the amount of strain and thus        the maximum size of the QDs formed thereon. For example, the        analog graded quantum well layers 1031, 1047, 1067, 1083 can        have a maximum In concentration of 36-40% (more preferably 38%)        relative to the concentration of Ga, which is suitable for QD        sizes to support 1550 nm emission. In another example, the        analog graded quantum well layers 1031, 1047, 1067, 1083 can        have a maximum In concentration of 30-36% (more preferably 33%)        relative to the concentration of Ga, which is suitable for QD        sizes to support 1430 nm emission. In yet another example, the        analog graded quantum well layers 1031, 1047, 1067, 1083 can        have a maximum In concentration of 27-33% (more preferably 30%)        relative to the concentration of Ga, which is suitable for QD        sizes to support 1310 nm emission. Other maximum In        concentrations for the analog graded quantum well layers 1031,        1047, 1067, 1083 (including maximum In concentrations relative        to Ga greater than 40%) can be used as well. It is contemplated        that maximum In concentrations relative to Ga greater than 40%        for the analog graded quantum well layers 1031, 1047, 1067, 1083        can reduce the amount of strain and thus increase the maximum        size of the QDs formed thereon.

An integrated circuit employing a wide variety of optoelectronic devicesand transistors can be made utilizing the layer structure of FIGS. 12Ato 12C. For all of the devices, n-type and p-type ion implants are usedto contact the n-type and p-type modulation doped QW structures,respectively. N-type metal is used to contact to the n-type ion implantsand the bottom n-type ohmic layer. P-type metal is used to contact tothe p-type ion implants and the top p-type ohmic layer.

FIGS. 15A-15C shows a straight passive waveguide section that operatespassively to guide light produced by a quantum well laser of the typethat is described above with respect to FIG. 5 and shown in more detailin the cross-section of FIG. 15B. The straight passive waveguide sectionis shown in more detail in the cross-section of FIG. 15C. The straightpassive waveguide section employs a top DBR mirror 1230 (preferablyrealized from pairs of semiconductor or dielectric materials withdifferent refractive indices) that operates as cladding to provideguiding of the optical mode 1203 between the top DBR mirror 1230 and thebottom DBR mirror formed by the periodic structure of layers 1003 and1005. The lateral confinement of the optical mode 1203 is provided bythe index change associated with vertical sidewalls 1213 of top rib aswell as n-type ion implants 1215 that are subsequently formed asdescribed below. The lateral confinement of the optical mode 1203 canalso be supported by covering the sidewalls 1213, 1227 with the periodiclayer structure of the top DBR mirror 1230 (not shown). The n-typeimplants 1215 also introduce impurity free vacancy disordering into theadjacent waveguide core region when subjected to rapid thermalannealing. The bandgap of the disordered waveguide core region isincreased locally to substantially reduce absorption and associatedoptical loss. The top DBR mirror structure 1230 can also extend in acontinuous manner to form cladding over the active region of the quantumwell laser as is evident from the cross-section of FIG. 15B. In thisconfiguration, the top DBR mirror structure 1230 operates as claddingfor the optical mode 1203 generated by the quantum well laser betweenthe top DBR mirror structure 1230 and the bottom DBR mirror formed bythe periodic structure of layers 1003 and 1005. The lateral confinementof the optical mode 1203 is provided by the n-type ion implants 1207under the anode metal 1209 of the quantum well laser that aresubsequently formed as described below. The size of the embedded QDs ofthe template and emission substructures of layers 1059-1087 1023-1051correspond to the desired emission/absorption wavelength of the opticalmode 1203 emitted by the quantum well laser.

FIG. 16 shows a configuration of a quantum well laser of the type thatis described above with respect to FIG. 8. This configuration can beutilized in conjunction with the straight passive waveguide section ofFIGS. 15A and 15C in a manner similar to the quantum well laser of FIG.15B. For the quantum well laser, the top DBR mirror structure 1230 canextend in a continuous manner to form cladding over the active region ofthe quantum well laser as is evident from the cross-section of FIG. 16.In this configuration, the top DBR mirror structure 1230 operates ascladding for the optical mode 1205 generated by the quantum well laserbetween the top DBR mirror structure 1230 and the bottom DBR mirrorformed by the periodic structure of layers 1003 and 1005. The lateralconfinement of the optical mode 1205 is provided by the n-type ionimplants 1207 that are subsequently formed in the top p-type region ofthe device as described below. The size of the embedded QDs of thetemplate and emission substructures of layers 1023-1051 correspond tothe desired emission/absorption wavelength of the optical mode 1205emitted by the quantum well laser.

FIG. 17 shows a configuration of a thyristor laser of the type that isdescribed above with respect to FIG. 9. This configuration can beutilized in conjunction with the straight passive waveguide section ofFIGS. 15A and 15C similar to the quantum well lasers of FIGS. 15B and16. For the thyristor laser, the top DBR mirror structure 1230 canextends in a continuous manner to form cladding over the active regionof the thyristor laser as is evident from the cross-section of FIG. 17.In this configuration, the top DBR mirror structure 1230 operates ascladding for the optical mode 1206 generated by the thyristor laserbetween the top DBR mirror structure 1230 and the bottom DBR mirrorformed by the periodic structure of layers 1003 and 1005. The lateralconfinement of the optical mode 1205 is provided by the n-type ionimplants 1207 that are subsequently formed under the top anode metal asdescribed below. The size of the embedded QDs of the template andemission substructures of layers 1059-1087 and 1023-1051 correspond tothe desired emission/absorption wavelength of the optical mode 1206emitted by the quantum well laser.

FIGS. 18A-18F illustrate a configuration of an optical closed loopmicroresonator that can be made utilizing the layer structure of FIGS.12A to 12C, which includes a microresonator 2000 spaced from a sectionof a zig-zag waveguide structure 2001 by a gap region G. The zig-zagwaveguide structure 2001 is optically coupled to the microresonator 2000by evanescent-wave coupling over the gap region G. The microresonator2000 defines a waveguide 2002 that follows a closed path that isgenerally rectangular in shape. The length of the closed path waveguide2002 is tuned to the particular wavelength of the optical mode 2004 thatis to propagate in the waveguide 2002. Specifically, the length of therectangular closed path waveguide 2002 is given as 2(L₁+L₂) for the L₁and L₂ length parameters of the waveguide 2002 as best shown in FIG.18B. In this configuration, the L₁ and L₂ parameters are selected toconform to the following:

$\begin{matrix}{{2\left( {L_{1} + L_{2}} \right)} = \frac{2\pi\; m\;\lambda_{D}}{n_{eff}}} & (1)\end{matrix}$

where L₁ and L₂ are the effective lengths of the opposed sides of theclosed path waveguide 2002;

-   -   m is an integer greater than zero;    -   λ_(D) is the wavelength of the optical mode 2004 that is to        propagate in the waveguide 2002; and    -   n_(eff) is the effective refractive index of the waveguide 2002.        The size of the embedded QDs of the template and emission        substructures of layers 1059-1087 (and possibly also layers        1023-1051) correspond to the desired wavelength λ_(D).

The width (W) of the closed path waveguide 2002 can be less than 2 μm,and possibly 1 μm or less. The width of the gap region G (i.e., thespacing between the waveguide 2002 and the zig-zag waveguide 2001) canbe less than 2 μm, and possibly on the order of 1 μm or less.

The optical mode 2004 circulates around the waveguide 2002 and isstrongly confined within the waveguide 2002 by internal reflection atthe reflective interfaces of the waveguide 2002. Specifically, claddingfor guiding the optical mode 2004 in the waveguide 2002 is provided bythe top DBR mirror structure 1230 and the bottom DBR mirror defined bythe periodic structure of layers 1003 and 1005 as best shown in thecross-section of FIG. 18C. Lateral confinement of the optical mode 2004in the waveguide 2002 can be provided by: i) a refractive index changeat the sidewalls 2006, 2007, 2021 that define the outer boundary of thewaveguide 2002 (FIG. 18A), ii) a refractive index change at the cornersidewalls 2006 (FIG. 18A), iii) a refractive index change at theperiphery of the implant regions 1215, 1221 located adjacent thesidewalls 2007, 2021 of the rib waveguide 2002 as evident from FIG. 18C,iv) a refractive index change at the periphery of the central implantregion 2012 located under the top anode electrode 2051 as evident fromFIGS. 18C and 18D, and v) a refractive index change at the interface ofthe top mirror 1230 that covers the sidewalls 2006, 2007, and 2021 asshown.

The zig-zag waveguide 2001 of FIGS. 18A to 18F defines a rib waveguide2008 that forms a zig-zag path. The optical mode 2010 is stronglyconfined within the waveguide 2008 by internal reflection at thereflective interfaces of the waveguide 2008. Specifically, cladding forguiding the optical mode 2010 in the waveguide 2008 can be provided bythe top DBR mirror structure 1230 and the bottom DBR mirror defined bythe periodic structure of layers 1003 and 1005 as best shown in thecross-section of FIG. 18D. Lateral confinement of the optical mode 2010in the waveguide 2008 can be provided by i) a refractive index change atthe top rib defined by the sidewalls 2019 that define the outer boundaryof the waveguide 2008 (FIG. 18A), ii) a refractive index change atn-type ion implants 1215 adjacent the top rib sidewalls similar to then-type implant 1215 shown in FIG. 18D for the microresonator waveguide2002, iii) a refractive index change at the corner sidewalls 2018 (FIG.18A), and iv) a refractive index change at the interface of the topmirror 1230 that covers the sidewalls 2018 and 2019.

In the coupling region, the waveguide 2008 includes a section thatextends parallel to and is closely-spaced from a straight section of themicroresonator waveguide 2002 by the gap region G. In this section ofthe waveguide 2008, lateral confinement of the optical mode 2010 isprovided by a refractive index change at the periphery of the implantregion 2014 under the metal 1209 of the first control electrode 2059 asshown in FIG. 18D, and a refractive index change at the periphery of theimplant region 2016 in the coupling region (gap G) as evident from FIG.18D. In the coupling section of the microresonator waveguide 2002,lateral confinement of the optical mode 2004 is further provided by arefractive index change at the periphery of the implant region 2016 inthe coupling region (gap G) as evident from FIG. 18D.

The microresonator 2000 further includes a top anode terminal electrode2051 that is electrically coupled to the top p-type ohmic contact layer(layer 1107) as best shown in FIGS. 18C and 18D, a cathode terminalelectrode 2053 that is electrically coupled to the n-type modulationdoped QW structure (layers 1097-1091) via an n-type ion implants 1215 asbest shown in FIG. 18C, a p-channel terminal electrode 2055 that iselectrically coupled to the p-type modulation doped QW structure (layers1019-1015) via a p-type ion implants 1221 as best shown in FIG. 18C, anda bottom contact terminal electrode 2057 that is electrically coupled tothe bottom n-type contact layer (layer 1009) as best shown in FIG. 18D.The zig-zag waveguide structure 2001 includes first control electrode2059 that is electrically coupled to the top p-type ohmic contact layer(layer 1107) as best shown in FIG. 18D, and a second control terminalelectrode 2061 that is electrically coupled to the n-type modulationdoped QW structure (layers 1097-1091) via an n-type ion implant region1215 as best shown in FIG. 18D. Note that the implant regions 2012,2014, and 2016 can locally shift the band gap in the underlying n-typemodulation doped quantum well structure (layers 1097-1091). This bandgapshift can prohibit charge transfer in the QWs of the n-type modulationdoped QW structure (layers 1097-1091) across the gap region G betweenthe adjacent waveguides 2002, 2008. For the waveguide 2008, voltagesignals applied to the top control electrode 2059 can overcome thiseffect to allow charge to enter (or exit) from the QWs of the n-typemodulation doped QW structure (layers 1097-1091) via the second controlelectrode 2061 as desired. It is also contemplated that additionalprocess steps, such as etching away the top p+ contact layers (layers1107-1105) and possibly additional layers thereunder in the gap regionG, can be performed in order to prevent any charge transfer across thegap region G.

The optical closed loop microresonator of FIGS. 18A to 18E can beconfigured as an electrically-pumped in-plane laser by forward biasingthe anode terminal electrode 2051 of the microresonator 2000 relative tothe cathode terminal electrode 2053 while allowing the p-channelelectrode 2055 and the bottom contact electrode 2057 to float. The biasconditions of the anode terminal electrode 2051 and the cathode terminalelectrode 2053 are configured to induce current flow into the activewaveguide region of the device sufficient to produce lasing action. Suchlasing action is controlled in a similar manner as described above withrespect to the electrically-pumped laser of FIG. 5. In thisconfiguration, the size of the embedded QDs of the template and emissionsubstructures of layers 1059-1087 (and possibly also layers 1023-1051)correspond to the desired wavelength λ_(D). With these bias conditions,the microresonator 2000 generates a continuous-wave optical signal atthe desired wavelength λ_(D) that propagates clockwise in the waveguide2002. Concurrent with such operation, a time-varying differentialelectrical signal is supplied to the first control electrode 2059 andthe second control electrode 2061 of the waveguide structure 2001 tochange the amount of charge (electrons) that fills the n-type modulationdoped QW structure (layers 1097-1091) for the waveguide 2008 whichinduces a change in the refractive index of the material of the n-typemodulation doped QW structure for the waveguide 2008. The change in therefractive index of the material of the n-type modulation doped QWstructure for the waveguide 2008 modulates the coupling coefficient forthe waveguide 2008 to cause modulation of the evanescent-wave couplingbetween the two waveguides 2002, 2008 in the coupling region G overtime. Specifically, the coupling coefficient for the waveguide 2008 (andthus the evanescent-wave coupling between the two waveguides 2002, 2008in the coupling region G) is controlled by the amount of charge(electrons) that fills the n-type modulation doped QW structure (layers1097-1091) for the waveguide 2008, which dictates the shifting of theabsorption edge of the QW(s) of the n-type modulation doped QW structurefor the waveguide 2008. The n-type modulation doped QW structure (layers1097-1091) for the waveguide 2008 can be filled with charge (electrons)by forward biasing the first control electrode 2059 with respect to thesecond control electrode 2061 under conditions where there is minimalconduction between the first control electrode 2059 and the secondcontrol electrode 2061. In this configuration, the first controlelectrode 2059 makes contact to the p-type ohmic contact layer whichextends over the waveguide 2008. The isolation implant 2014 populatesthe QW channel of the n-type modulation doped QW structure due to itseffect on the modulation doping. Effectively this moves the contact tothe point in the waveguide 2008 on the other side of the isolationimplant 2014. The forward bias condition of the first control electrode2059 with respect to the second control electrode 2061 results inminimal conduction between the first control electrode 2059 and thesecond control electrode 2061 while producing a field effect thatcontrols the charge (electrons) that fills the n-type modulation dopedQW structure (layers 1097-1091) for the waveguide 2008. Such biasoperations are similar to the bias conditions of the n-channel HFET informing the 2-dimensional electron gas for the operation of then-channel HFET as described above. Since the charge is now in thewaveguide, the absorption edge of the QW of the n-type modulation dopedQW structure for the waveguide 2008 shifts to change the couplingcoefficient for the waveguide 2008.

Such coupling modulation generates a modulated optical signal based uponthe continuous-wave optical signal that propagates clockwise in thewaveguide 2002. The modulated optical signal propagates from thecoupling region of the waveguide 2008 and is output from the waveguide2008 as best shown in FIGS. 18A and 18B. The modulated optical signaloutput from the waveguide 2008 can have an optical OOK modulation format(i.e., digital pulsed-mode optical signal) or possibly a higher orderoptical modulation format (such as optical differential phase shiftkeying format or optical differential quadrature phase shift keyingformat). The bias conditions of the anode terminal electrode 2051 andthe cathode terminal electrode 2053 as well as the electrical signalssupplied to the first and second control electrodes 2059, 2061 can beprovided by resistors and/or transistors integrated on-chip (i.e., onthe substrate 1001) or off-chip. Advantageously, it is expected that theelectronic control of the evanescent coupling between the waveguide 2002and the waveguide 2008 can modulate the optical signal output from thewaveguide 2008 at high bandwidths, which can possibly extend up to 100GHz.

For continuous-wave emission of the laser, a DC differential electricalsignal can be supplied to the first and second control electrodes 2059,2061 of the waveguide structure 2001 (instead of the time-varying signalfor the modulated emission). The DC electrical signal controls thedevice to operate in the coupled state. In this coupled state, thecontinuous-wave optical signal that propagates clockwise in thewaveguide 2002 is transferred to the waveguide 2008 (in the couplingregion of waveguide 2008) and is output from the waveguide 2008.

The characteristic wavelength λ_(D) of the continuous-wave opticalsignal that propagates clockwise in the waveguide 2002 can be tuned bycontrolled heating of the device to control the temperature of themicroresonator 2000. Such controlled heating can be carried out bylocalized heating through controlled operation of a transistor device(such as n-channel or p-channel HFET device) that is integrally formedon the substrate in a position adjacent to or near the microresonator2000. The transistor device is operated as a resistive heater togenerate heat in a controlled manner. The heat transfers by diffusion toheat the microresonator 2000. Temperature of the microresonator 2000 (orof a device in the local vicinity thereof) can be used to providefeedback to control the heating of the microresonator 2000 via thetransistor heater device. The heating current of the heater transistordevice for controlling the optical closed loop microresonator to emitlight at the desired characteristic wavelength λ_(D) can be measured byspectral analysis of the wavelength of the light output from the opticalclosed loop microresonator as compared to the desired characteristicwavelength λ_(D) and adjusting the heating current of the heatertransistor device such that the wavelength of the output light matchesthe desired characteristic wavelength λ_(D). The spectral analysis canbe accomplished by using a 4-port directional coupler with 2 ports (aninput port and output port) for two waveguides. The two waveguides ofthe directional coupler are configured to couple an optical signal atthe desired characteristic wavelength λ_(D) that is supplied to theinput port of one waveguide to the other waveguide for output therefrom,and vice versa. Light output by the optical closed loop microresonatoris tapped off and fed into the input port of one of the waveguides ofthe directional coupler. The light output of a reference source thatoperates at the desired characteristic wavelength λ_(D) is fed into theinput port of the other waveguide of the directional coupler. The lightoutput of the two output ports of the waveguides of the directionalcoupler is directed to two photodetectors, which are arranged asbalanced detectors that produce an output signal proportional to thedifference between the wavelength of the light output by the opticalclosed loop microresonator and the desired characteristic wavelengthλ_(D) for the light output by the reference source. This output signalis used in a feedback loop that dynamically adjusts the heating currentof the heater transistor device (which changes the wavelength of thelight output by the optical closed loop microresonator) to drive theoutput signal to near zero such that the wavelength of the output by theoptical closed loop microresonator matches the desired characteristicwavelength λ_(D).

In the preferred embodiment, the microresonator 2000 as well as thezig-zag waveguide structure 2001 and the transistor heater device areall formed over a continuous section of the bottom DBR mirror (andpossibly a number of layers above the bottom DBR mirror). This isrealized by omitting the isolation etch through the bottom DBR mirrorfor the region between the microresonator 2000 and the transistor heaterdevice (or portions thereof). This configuration allows the heatgenerated by the transistor heater device to diffuse through thiscontinuous section of the bottom DBR mirror (and possibly through anumber of layers above the bottom DBR mirror) to the microresonator 2000for the desired heating. FIG. 18E illustrates an exemplary configurationwhere the microresonator 2000 and a heating n-channel HFET device 2075are both formed over a continuous section of the bottom mirror. Theoperation of the re-channel HFET device 2075 is described above withrespect to FIG. 1. Note that the isolation etch through the bottom DBRmirror is omitted for the region between the microresonator 2000 and thetransistor heater device 2075 (or portions thereof) in order to allowthe heat generated by the transistor heater device 2075 to diffusethrough this continuous section of the bottom DBR mirror to themicroresonator 2000 for the desired heating. The heat generated by thetransistor heater device 2075 can be controlled electronically bybiasing the transistor heater device 2075 to control the conductance ofthe channel of the transistor heater device 2075 and by controlling theamount of current that flows through the channel between the sourceterminal electrode 53 and the drain terminal electrode 55 in theconducting (ON) state of the transistor heater device 2075. Othersuitable control operations can be implemented for other designs. Thelayout for the transistor heater device 2075 can be different from thatshown in FIG. 18E. For example, a long channel design or serpentinechannel design can be used.

The characteristic wavelength λ_(D) of the continuous-wave opticalsignal that propagates clockwise in the waveguide 2002 can also be tunedby changing the effective length of the optical path of the waveguide2002. This can be accomplished with the addition of one or moreclosed-path waveguides (two shown as 2081A, 2081B) that are evanescentlycoupled to the closed-path waveguide 2002 over a second gap region G2 asshown in FIG. 18F.

The closed path waveguides 2081A, 2081B each follow a closed path thatis generally rectangular in shape, which allow for circulation of acorresponding optical mode signal around the respective closed-pathwaveguide. The optical modes are strongly confined within theclosed-path waveguides by internal reflection at the reflectiveinterfaces of the closed-path waveguides similar to the zig-zagwaveguide 2008. Specifically, cladding for guiding the optical mode inthe respective closed-path waveguide 2081A or 2081B is provided by thetop DBR mirror structure 1230 and the bottom DBR mirror defined by theperiodic structure of layers 1003 and 1005 similar to confinement of theoptical mode 2010 as shown in the cross-section of FIG. 18D. Lateralconfinement of the optical mode in the respective closed-path waveguide2081A or 2081B can be provided by refractive index changes at the toprib defined by the sidewalls that define the outer boundary of therespective waveguide (similar to the sidewalls shown in FIG. 18A), atn-type ion implants 1215 adjacent the top rib sidewalls similar to then-type implant 1215 shown in FIG. 18D for the microresonator waveguide2002, at the corner sidewalls 2018 (similar to the corner sidewallsshown in FIG. 18A), and at the interface of the top mirror 1230 thatcovers the sidewalls 2018 and 2019. In their respective coupling region,the closed-path waveguide 2081A or 2081B includes a section that extendsparallel to and is closely-spaced from a straight section of themicroresonator waveguide 2002 by the gap region G2. In this section ofthe waveguide 2081A or 2081B, lateral confinement of the optical mode isprovided by i) a refractive index change at the periphery of the implantregion under the metal 1209 of the first control electrode (2091A or2091B), which is similar to the implant region 2014 shown in FIG. 18D,and ii) a refractive index change at the periphery of the implant regionin the coupling region (gap G2), which is similar to the implant region1216 shown in FIG. 18D.

The closed path waveguides 2081A, 2081B each include a respective firstcontrol electrode (2091A, 2091B) that is electrically coupled to the topp-type ohmic contact layer (layer 1107) in a manner similar to thecontrol electrode 2059 shown in FIG. 18D, and a respective secondcontrol terminal electrode (2093A, 2093B) that is electrically coupledto the n-type modulation doped QW structure (layers 1097-1091) via ann-type ion implant region 1215 similar to the control electrode 2061shown in FIG. 18D. Note that implant regions similar to the implantregions 2014 and 2016 of FIG. 18D can be provided to locally shift theband gap in the underlying n-type modulation doped quantum wellstructure (layers 1097-1091). This bandgap shift can prohibit chargetransfer in the QWs of the n-type modulation doped QW structure (layers1097-1091) across the gap region G between the adjacent waveguides. Foreach respective closed path waveguides 2081A/2081B, voltage signalsapplied to the top control electrode 2091A/2091B3019 can overcome thiseffect to allow charge to enter (or exit) from the QWs of the n-typemodulation doped QW structure (layers 1097-1091) via the correspondingsecond control electrode 2093A, 2093B as desired. It is alsocontemplated that additional process steps, such as etching away the topp+ contact layers (layers 1107-1105) and possibly additional layersthereunder in the gap region G between the adjacent waveguides, can beperformed in order to prevent any charge transfer across the gap regionG between the adjacent waveguides.

Concurrent with the operation of the microresonator 2000 (and thezig-zag waveguide 2008), electrical signals are supplied to the firstand second control electrodes (2091A/2091B and 2093A, 2093B) for theclosed path waveguides 2081A, 2081B to control the amount of charge(electrons) that fills the n-type modulation doped QW structure (layers1097-1091) for the closed path waveguides 2081A, 2081B, which induces achange in the refractive index of the material of the n-type modulationdoped QW structure for the closed path waveguides 2081A, 2081B. Thechange in the refractive index of the material of the n-type modulationdoped QW structure for the closed path waveguides 2081A, 2081B modulatesthe coupling coefficient for the closed path waveguides 2081A, 2081B tocause modulation of the evanescent-wave coupling between the waveguide2002 and the closed path waveguides 2081A and 2081B in the couplingregion G2 over time. The change in such coupling coefficients changesthe effective optical path of the waveguide 2002, which can be used totune the characteristic wavelength λ_(D) of the continuous-wave opticalsignal that propagates clockwise in the waveguide 2002. The controlsignals for the coupling between the waveguide 2002 and the closed pathwaveguides 2081A and 2081B that controls the optical closed loopmicroresonator 2000 to emit light at the desired characteristicwavelength λ_(D) can be measured by spectral analysis of the wavelengthof the light output from the optical closed loop microresonator ascompared to the desired characteristic wavelength λ_(D) and adjustingthe control signals for the coupling between the waveguide 2002 and theclosed path waveguides 2081A and such that the wavelength of the outputlight matches the desired characteristic wavelength λ_(D). The spectralanalysis can be accomplished by using a 4-port directional coupler with2 ports (an input port and output port) for two waveguides. The twowaveguides of the directional coupler are configured to couple anoptical signal at the desired characteristic wavelength λ_(D) that issupplied to the input port of one waveguide to the other waveguide foroutput therefrom, and vice versa. Light output by the optical closedloop microresonator is tapped off and fed into the input port of one ofthe waveguides of the directional coupler. The light output of areference source that operates at the desired characteristic wavelengthλ_(D) is fed into the input port of the other waveguide of thedirectional coupler. The light output of the two output ports of thewaveguides of the directional coupler is directed to two photodetectors,which are arranged as balanced detectors that produce an output signalproportional to the difference between the wavelength of the lightoutput by the optical closed loop microresonator and the desiredcharacteristic wavelength λ_(D) for the light output by the referencesource. This output signal is used in a feedback loop that dynamicallyadjusts the control signals for the coupling between the waveguide 2002and the closed path waveguides 2081A and 2081B (which changes theeffective optical path of the waveguide 2002 and thus the wavelength ofthe light output by the optical closed loop microresonator) to drive theoutput signal to near zero such that the wavelength of the output by theoptical closed loop microresonator matches the desired characteristicwavelength λ_(D).

The optical closed loop microresonator of FIGS. 18A to 18F can also beconfigured as an in-plane laser by forward biasing the p-channelinjector terminal electrode 2055 of the microresonator 2000 relative tothe bottom contact electrode 2057 while allowing the n-channel injectorelectrode 2053 and the top contact electrode 2051 to float. The biasconditions of the anode terminal electrode 2051 and the cathode terminalelectrode 2053 are configured to induce current flow into the activewaveguide region of the device sufficient to produce lasing action. Suchlasing action is controlled in a similar manner as described above withrespect to the quantum well laser of FIG. 8. In this configuration, thesize of the embedded QDs of the template and emission substructures oflayers 1023-1051 correspond to the desired wavelength λ_(D). Concurrentwith such operation, a time-varying differential electrical signal canbe supplied to the first control electrode 2059 and the second controlelectrode 2061 of the waveguide structure 2001 to change the couplingcoefficient for the waveguide 2008 and modulate the evanescent couplingbetween the waveguides 2002 and 2008. Alternately, for continuousoutput, a DC electrical signal can be supplied to the first controlelectrode 2059 and the second control electrode 2061 of the waveguidestructure 2001 to activate the evanescent coupling between thewaveguides 2002 and 2008. The characteristic wavelength λ_(D) of thecontinuous-wave optical signal that propagates clockwise in thewaveguide 2002 can be tuned by controlling the bias conditions appliedto the microresonator 2000 to adjust the charge (electrons) that fillthe quantum well(s) of the n-type modulation doped QW structure (layers1097-1091) for the waveguides 2002 and/or 2008, and or by controlledheating of the device to control the temperature of the microresonator2000.

The optical closed loop microresonator of FIGS. 18A-18F can also beconfigured for optical-to-electrical conversion of an input opticalsignal supplied to the zig-zag waveguide structure 2001. In this case,the input optical signal is supplied to the waveguide 2008 andevanescent-wave coupling between the two waveguides 2008, 2002 in thecoupling region G is activated to such that input optical signal istransmitted from the waveguide 2008 to the waveguide 2002 to generatethe optical signal propagating in the waveguide 2002. Theevanescent-wave coupling between the two waveguides 2008, 2002 in thecoupling region G can be activated (and deactivated) by controlling theamount of charge (electrons) that fills the n-type modulation doped QWstructure (layers 1097-1091) for the waveguides 2002 and 2008, whichdictates the shifting of the absorption edge and index of refraction ofthe QW(s) of the n-type modulation doped QW structure for the waveguides2002, 2008. Charge can be added to (or removed from) the n-typemodulation doped QW structure (layers 1097-1091) for the waveguide 2008by a suitable bias current source and/or bias current sink that iselectrically coupled to the second control electrode 2061. Similarly,charge can be added to (or removed from) the n-type modulation doped QWstructure (layers 1097-1091) for the waveguide 2002 by a suitable biascurrent source and/or bias current sink that is electrically coupled tothe re-channel contact terminal electrode (i.e., the cathode terminalelectrode 2053 of FIGS. 18A-18E) of the microresonator 2000.

In one embodiment suitable for optical-to-electrical conversion, themicroresonator 2000 can be configured for thyristor operation where theoptical signal propagating in the waveguide 2002 generates photocurrentby absorption which adds electrons to the n-type modulation doped QWstructure (layers 1097-1091) and holes to the p-type modulation doped QWstructure (layers 1019-105) such that the thyristor device switches ONand conducts current through the device between the anode terminalelectrode 2051 and the thyristor cathode (i.e., the bottom contactelectrode 2057 of FIGS. 18A-18E). Such optoelectronic operations providethe function of detection, current-to-voltage conversion (typicallyprovided by a transimpedance amplifier), level shifting to obtain aground reference and a decision circuit (typically realized by acomparator). Moreover, the microresonator 2000 has an advantage that itwill only absorb at the resonator frequency and thus can be adapted tosupport different wavelengths for wavelength division multiplexingapplications. In this configuration, the size of the embedded QDs of thetemplate and emission substructures of layers 1097-1091 and/or layers1023-1051 correspond to the desired wavelength λ_(D).

In another embodiment suitable for optical-to-electrical conversion, themicroresonator 2000 can be configured for optical detection analogous toa photodiode by applying a reverse bias between the anode terminal 2051and the n-channel contact electrode (the cathode terminal electrode 2053of FIGS. 18A-18F). In this configuration, the optical signal propagatingin the waveguide 2002 generates photocurrent by absorption of theembedded QDs of the template and emission substructures of layers1097-1091, which flows between the anode terminal 2051 and the n-channelcontact electrode (the cathode terminal electrode 2053 of FIGS.18A-18E). In this configuration, the size of the embedded QDs of thetemplate and emission substructures of layers 1097-1091 correspond tothe desired wavelength λ_(D). Similar operations can by applying areverse bias between the p-channel injector terminal 2055 and the bottomcontact electrode 2057 of FIGS. 18A-18F. In this configuration, theoptical signal propagating in the waveguide 2002 generates photocurrentby absorption of the embedded QDs of the template and emissionsubstructures of layers 1023-1051, which flows between the p-channelinjector terminal 2055 and the bottom contact electrode 2057. In thisconfiguration, the size of the embedded QDs of the template and emissionsubstructures of layers 1023-1051 correspond to the desired wavelengthλ_(D)

In another embodiment suitable for optical-to-electrical conversion, themicroresonator 2000 can be configured for optical detection analogous toan n-channel base BICFET phototransistor device by applying biasconditions to the terminals of the device (the emitter terminalelectrode which corresponds to the anode terminal electrode 2051, thebase terminal electrode which corresponds to the cathode terminalelectrode 2053, and the collector terminal electrode which correspondsto the p-channel contact electrode 2055 of FIGS. 18A-18F) for constantcurrent operation. In this configuration, the optical signal propagatingin the waveguide 2002 generates photocurrent by absorption of theembedded QDs of the template and emission substructures of layers1023-1051 and layers 1097-1091, which adds to the base current and thusto the collector current (and the emitter current) of the device. Inthis configuration, the size of the embedded QDs of the template andemission substructures of layers 1023-1051 and layers 1097-1091correspond to the desired wavelength λ_(D). Similar operations can beconfigured for optical detection analogous to a p-channel base BICFETphototransistor device by applying bias conditions to the terminals ofthe device (the emitter terminal electrode which corresponds to thebottom contact electrode 2057, the base terminal electrode whichcorresponds to the p-channel electrode 2055, and the collector terminalelectrode which corresponds to the n-channel electrode 2053 of FIGS.18A-18F) for constant current operation. In this configuration, theoptical signal propagating in the waveguide 2002 generates photocurrentby absorption of the embedded QDs of the template and emissionsubstructures of layers 1023-1051 and layers 1097-1091, which adds tothe base current and thus to the collector current (and the emittercurrent) of the device. In this configuration, the size of the embeddedQDs of the template and emission substructures of layers 1023-1051 andlayers 1097-1091 correspond to the desired wavelength λ_(D).

The optical closed loop microresonator of FIGS. 18A-18F can also beconfigured for other optoelectronic functions, such as modulation of aninput optical signal supplied to the zig-zag waveguide structure 2001for output from the zig-zag waveguide structure 2001 as well as opticalswitching of an input optical signal supplied to the zig-zag waveguidestructure 2001. Such configurations are similar to those described indetail in International Appl. No. PCT/US12/51265 filed on Aug. 17, 2012and published as WO2013/025964 on Feb. 21, 2013, commonly assigned toassignee of the present application and herein incorporated by referencein its entirety.

FIGS. 19A-19C illustrate a configuration of a waveguide optical couplerthat can be made utilizing the layer structure of FIGS. 12A to 12C,which includes two zig-zag active waveguide structures 3001, 3003integrated on the substrate 1001 and optically coupled to one another byevanescent-wave coupling over a gap region G. The zig-zag waveguidestructure 3001 is defined by a rib waveguide 3005 that forms a zig-zagpath. Similarly, the zig-zag waveguide structure 3003 is defined by arib waveguide 3007 that forms a zig-zag path. The optical mode thattravels through each respective rib waveguide is strongly confinedwithin the respective rib waveguide by internal reflection at thereflective interfaces of the rib waveguide. Specifically, cladding forguiding the optical mode 3009 in the rib waveguide 3007 is provided bythe top DBR mirror 1230 and the bottom DBR mirror defined by theperiodic structure of layers 1003 and 1005 as best shown in thecross-section of FIG. 19B. Lateral confinement of the optical mode 3009in the waveguide 3007 is provided by refractive index changes at thesidewalls 3011, 3013 that define the outer boundary of the waveguide3007 (FIG. 19A), at n-type ion implants 1215 adjacent the top ribsidewalls 3011 (FIG. 19B), at the corner sidewalls, and at the interfaceof the top mirror 1230 that covers the sidewalls 3011. Similar structureis used for the upper cladding of waveguide 3005 for guiding the opticalmode 3015 in the rib waveguide 3005.

In the coupling region, the waveguides 3005 and 3007 include straightsections that extend parallel to one another and closely-spaced from oneanother by the gap region G. In the straight section of the waveguide3005, vertical confinement of the optical modes 3015, 3009 in thewaveguides 3005, 3007 can be aided by the top mirror 1230 formed tocover the top and sidewalls 3011, 3013 of the waveguides 3005, 3007 asshown. Lateral confinement of the optical mode 3015 is provided by i) arefractive index change at the periphery of the implant region 3017under the metal 1209 of the top control electrode 3019 as shown in FIG.19C, and ii) a refractive index change at the periphery of the implantregion 3021 in the coupling region (gap G) as evident from FIG. 19C. Inthe straight section of the waveguide 3007, lateral confinement of theoptical mode 3009 is provided by i) a refractive index change at theperiphery of the implant region 3023 under the metal 1209 of the topcontrol electrode 3025 as shown in FIG. 19C, and ii) a refractive indexchange at the periphery of the implant region 3021 in the couplingregion (gap G) as evident from FIG. 19C.

The width (W) of the waveguides 3005, 3007 can be less than 2 μm, andpossibly 1 μm or less. The width of the gap region G (i.e., the spacingbetween the waveguides 3007, 3009) can be less than 2 μm, and possiblyon the order of 1 μm or less.

The zig-zag active waveguide structure 3001 includes a top controlterminal electrode 3019 that is electrically coupled to the top p-typeohmic contact layer (layer 1107) as best shown in FIG. 19C, a secondcontrol terminal electrode 3027 that is electrically coupled to then-type modulation doped QW structure (layers 1097-1091) via an n-typeion implant 1215 as best shown in FIG. 19C, and a bottom contactterminal electrode 3029 that is electrically coupled to the bottomn-type contact layer (layer 1009) as best shown in FIG. 19C. The zig-zagactive waveguide structure 3003 includes a top control terminalelectrode 3025 that is electrically coupled to the top p-type ohmiccontact layer (layer 1107) as best shown in FIG. 19C, a second controlterminal electrode 3031 that is electrically coupled to the n-typemodulation doped QW structure (layers 1097-1091) via an n-type ionimplant 1215 as best shown in FIG. 19C, and a bottom contact terminalelectrode 3033 that is electrically coupled to the bottom n-type contactlayer (layer 1009) as best shown in FIG. 19C. Note that the implantregions 3017, 3021, 3023 can locally shift the band gap in theunderlying n-type modulation doped quantum well structure (layers1097-1091). This bandgap shift can prohibit charge transfer in the QWsof the n-type modulation doped QW structure (layers 1097-1091) acrossthe gap region G between the adjacent waveguides 3005, 2007. For thewaveguide 3005, voltage signals applied to the top control electrode3019 can overcome this effect to allow charge to enter (or exit) fromthe QWs of the n-type modulation doped QW structure (layers 1097-1091)via the corresponding second control electrode 3027 as desired. For thewaveguide 3007, voltage signals applied to the top control electrode3025 can overcome this effect to allow charge to enter (or exit) fromthe QWs of the n-type modulation doped QW structure (layers 1097-1091)via the corresponding second control electrode 3031 as desired. It isalso contemplated that additional process steps, such as etching awaythe top p+ contact layers (layers 1107-1105) and possibly additionallayers thereunder in the gap region G between the adjacent waveguides3005, 3007, can be performed in order to prevent any charge transferacross the gap region G between the adjacent waveguides 3005, 3007.

The waveguide optical coupler of FIGS. 19A to 19C can be configured foroptical switching by biasing both bottom contact electrodes 3029, 3033to ground via load resistance and applying control signals to thecontrol electrodes 3019, 3027, 3025, 3031 in order to control theevanescent-wave coupling between the two waveguides 3005, 3007 in thecoupling region G. Specifically, the evanescent-wave coupling betweenthe two waveguides 3005, 3007 in the coupling region G can be activated(and deactivated) by controlling the amount of charge (electrons) thatfills the n-type modulation doped QW structure (layers 1097-1091) forthe waveguides 3005 and 3007, which dictates the shifting of theabsorption edge and the index of refraction of the QW(s) of the n-typemodulation doped QW structure for the waveguides 3005, 3007 over thelength of the coupling region G for the desired optical switching state(pass-thru state or switched state).

For the pass-thru state where the input optical signal is supplied tothe input of the waveguide structure 3001 and is output from waveguidestructure 3001 as best shown in FIG. 19A, it is required that the lightevanescently couple from waveguide 3005 to waveguide 3007 and then backto waveguide 3005 over the coupling region. There will be a wavelengthλ_(max) corresponding to the maximum shift of the absorption edge.According to Kramers Kronig relations, there will be an increase inrefractive index for λ>λ_(max), and a decrease in refractive index forλ<λ_(max). For this pass-thru state, the largest index is required andthus the λ of the input optical signal must be greater than λ_(max) ofthe device. This means that for this pass-thru state, the controlsignals to the control electrodes 3019, 3027, 3025, 3031 can beconfigured to fill the QWs of the n-type modulation doped QW structuresfor both waveguides 3005, 3007 with electrons. These conditions dictatethe shifting of the absorption edge and the index of refraction of theQW(s) of the n-type modulation doped QW structure for the waveguides3005, 3007 over the length of the coupling region G to cause the lightof the input optical signal to evanescently couple from waveguide 3005to waveguide 3007 and then back to waveguide 3005 over the couplingregion for output from waveguide structure 3001. For the waveguide 3005,the bias conditions for the pass-thru state can be realized by applyinga forward bias of the top control electrode 3019 with respect to thesecond control electrode 3027 which results in minimal conductionbetween the top control electrode 3019 and the second control electrode3027 while producing a field effect that fills the n-type modulationdoped QW structure (layers 1097-1091) for the waveguide 3005 with charge(electrons). Such bias operations are similar to the bias conditions ofthe n-channel HFET in forming the 2-dimensional electron gas for theoperation of the re-channel HFET as described above. For the waveguide3007, the bias conditions for the pass-thru state can be realized byapplying a forward bias of the top control electrode 3025 with respectto the second control electrode 3031 which results in minimal conductionbetween the top control electrode 3025 and the second control electrode3031 while producing a field effect that fills the n-type modulationdoped QW structure (layers 1097-1091) for the waveguide 3007 with charge(electrons). Again, such bias operations are similar to the biasconditions of the n-channel HFET in forming the 2-dimensional electrongas for the operation of the n-channel HFET as described above. It isalso contemplated that the pass-thru state can be configured bysupplying control signals to the control electrodes 3019, 3027, 3025,3031 that empty charge from the QWs of the n-type modulation doped QWstructures for both waveguides 3005, 3007. With the QWs of the n-typemodulation doped QW structures for both waveguides 3005, 3007 bothfilled with or emptied of charge, there is no index difference betweenthe waveguides 3005, 3007 and the pass-thru state is obtained.

For the switched state where the input optical signal is supplied to theinput of the waveguide structure 3001 and is output from waveguidestructure 3003 as best shown in FIG. 19A, it is required that the lightevanescently couple from waveguide 3005 to waveguide 3007 over thecoupling region (without coupling back to the waveguide 3005). For thisswitched state, the control signals to the control electrodes 3019,3027, 3025, 3031 can be configured to fill only the QWs of the n-typemodulation doped QW structure (layers 1097-1091) of waveguide 3005 withelectrons, while emptying the QWs of the n-type modulation doped QWstructure (layers 1097-1091) of waveguide 3007 of electrons. Theseconditions produce a change in the absorption edge and refractive indexfor two waveguides 3005, 3007 that causes the light of the input opticalsignal to evanescently couple from waveguide 3005 to waveguide 3007 overthe coupling region (without coupling back to waveguide 3005) for outputfrom waveguide structure 3003. For the waveguide 3005, the biasconditions for the switched state can be realized by applying a forwardbias of the top control electrode 3019 with respect to the secondcontrol electrode 3027 which results in minimal conduction between thetop control electrode 3019 and the second control electrode 3027 whileproducing a field effect that fills the n-type modulation doped QWstructure (layers 1097-1091) for the waveguide 3005 with charge(electrons). Such bias operations are similar to the bias conditions ofthe n-channel HFET in forming the 2-dimensional electron gas for theoperation of the n-channel HFET as described above. For the waveguide3007, the bias conditions for the switched state can be realized byapplying a reverse bias of the top control electrode 3025 with respectto the second control electrode 3031 which results in minimal conductionbetween the top control electrode 3025 and the second control electrode3031 while producing a field effect that empties charge (electrons) fromthe n-type modulation doped QW structure (layers 1097-1091) for thewaveguide 3007. It is also contemplated that the switched state can beconfigured by supplying control signals to the control electrodes 3019,3027, 3025, 3031 that fill only the QWs of the n-type modulation dopedQW structure (layers 1097-1091) of waveguide 3007 with electrons, whileemptying the QWs of the n-type modulation doped QW structure (layers1097-1091) of waveguide 3005 of electrons. With the QWs of the n-typemodulation doped QW structures for one of the waveguides (such as 3005)filled with electrons, and the QWs of the n-type modulation doped QWstructures for the other waveguide (such as 3007) emptied of electrons,there is an index difference between the waveguides 3005, 3007 and theswitched state is obtained.

The bias conditions of the device as well as the electrical signalssupplied to the control electrodes 3019, 3027, 3025, 3031 can beprovided by resistors and/or transistors integrated on-chip (i.e., onthe substrate 1001) or off-chip.

FIGS. 20A-20C illustrate straight passive waveguide sections 4001A,4001B that are disposed on the input side and output side of a waveguideoptical amplifier 4003 fabricated from the layer structure of FIGS. 12Ato 12C. The straight passive waveguide sections 4001A, 4001B operate topassively guide light into and from the waveguide optical amplifier4003. The waveguide optical amplifier 4003 is shown in more detail inthe cross-section of FIG. 20B. One of the straight passive waveguidesections (4001A) is shown in more detail in the cross-section of FIG.20C.

The straight passive waveguide sections 4001A, 4001B each employ a ribstructure defined by opposed sidewalls 1227. The optical mode thattravels through the respective rib waveguide structure is stronglyconfined within the respective rib waveguide structure by internalreflection at the reflective interfaces of the rib waveguide.Specifically, cladding for guiding the optical mode 4005 in the ribwaveguide 4001A is provided by the top DBR mirror 1230 and the bottomDBR mirror defined by the periodic structure of layers 1003 and 1005 asbest shown in the cross-section of FIG. 19B. Lateral confinement of theoptical mode 4005 in the waveguide 4001A is provided by refractive indexchanges at the sidewalls 1213 that define the outer boundary of thewaveguide 4001A (FIG. 20C), at n-type ion implants 1215 adjacent the toprib sidewalls similar to the passive waveguide sections of FIG. 15, andat the interface of the top mirror 1230 that covers the sidewalls 1213.Similar structure is used for the cladding of waveguide 4001B forguiding the optical mode 4005 in the rib waveguide 4001B. The top DBRmirror 1230 can extend in a continuous manner to form cladding over theaperture of the waveguide optical amplifier 4003 as is evident from thecross-section of FIG. 20B. In this configuration, the top DBR mirror1230 operates as cladding for the optical mode 4005 that propagatesthrough the waveguide optical amplifier 4003 between the top DBR mirror1230 and the bottom DBR mirror formed by the periodic structure oflayers 1003 and 1005. The lateral confinement of the optical mode 4005is provided by the n-type ion implants 1207 under the anode metal 1209of the waveguide optical amplifier 4003 that are subsequently formed asdescribed below.

The waveguide optical amplifier 4003 further includes a top anodeterminal electrode with two sections 4007A, 4007B that are electricallycoupled to the top p-type ohmic contact layer (layer 1107) on oppositesides of the active waveguide region defined by the waveguide structure1201 as best shown in FIGS. 20A and 20B. It also includes a cathodeterminal electrode with two sections 4009A, 4009B that are electricallycoupled to the n-type quantum well structure (layers 1097-1091) vian-type ion implants 1215 through opposite mesa regions 1211 outside theanode terminal sections 4007A, 4007B as best shown in FIGS. 20A and 20B.

The waveguide optical amplifier 4003 coupler of FIGS. 20A to 20C can beconfigured for optical amplification by forward biasing the anodeterminal electrode sections 4007A, 400B with respect to the cathodeterminal electrode sections 4009A, 4009B such that current flows betweenthe anode electrode sections 4007A, 4007B and the cathode electrodesections 4009A, 4009B of the device, but at a level below the lasingthreshold. In this configuration, the input optical signal that issupplied to the waveguide optical amplifier 4003 by the waveguidesection 4001 and travels along the optical path of the waveguide opticalamplifier 4003 is amplified in intensity by the respective device. Thebias conditions of the device can be provided by resistors and/ortransistors integrated on-chip (i.e., on the substrate 1001) oroff-chip. In this configuration, the size of the embedded QDs of thetemplate and emission substructures of layers 1097-1091 (and possiblythe layers 1023-1051) correspond to the wavelength of the input opticalmode to support the optical amplification function of the device.

The layers structure of FIGS. 12A-12C and the optoelectronic devices asdescribed above (as well as the sequence of fabrication steps describedbelow) can be adapted to realize other optoelectronic devices as part ofthe integrated circuit. For example, the device of FIGS. 20A-20C can beconfigured as an optical modulator with an optical path through theactive waveguide region of the device. An input signal is applied to theanode electrode such that the anode electrode is biased with respect tothe cathode electrode (which is electrically coupled to the n-typemodulation doped QW structure of the device) over a range of voltagelevels that produce an applied electric field that changes theabsorption of the device. In this configuration, the optical signal thatis supplied to the device and travels along the optical path of thedevice is modulated by the controlled time-varying absorption of thedevice. The modulation can be analog in nature by varying the electricfield in a linear fashion. Alternatively, the modulation can be digitalin nature by varying the electric field between two states: an on statewith limited optical loss through the optical path and an off state thatblocks the optical path through the active waveguide region of thedevice.

An integrated circuit employing the wide variety of optoelectronicdevices and transistors as described above in conjunction with the layerstructure of FIGS. 12A to 12C can be fabricated with a sequence ofoperations as follows.

First, alignment marks (not shown) are defined on the device structure.

Next, ion implantation of n-type ions is performed. The ion implantationcan employ a mask defined by lift-off of a double layer consisting of athin layer of silicon oxide (preferably 500 Å thick) and a thick layerof silicon nitride (preferably 2000 Å thick). The double layer maskdefines openings for the n-type ion implant regions.

The ion implantation can form n-type implant regions 1207 for theoptical devices as shown in FIGS. 15B, 16, 17, and 20B. The separationbetween such implant regions 1207 defines a waveguide region (oraperture) for the respective device. The implant regions 1207 can servetwo functions. First, the implants 1207 create a p-n junction betweenthe top p-type layers and the n-type implants that can funnel electricalp-type carriers (holes) injected from the top metal 1209 into thesection of QW channel of the n-type modulation doped QW structure(layers 1097-1091) that is positioned between and under the n-typeimplants 1207. Second, the implants 1207 are slightly lower in index sothat optical propagation is guided in the active waveguide regionbetween the implants 1207.

For the optical closed loop microresonator of FIGS. 18A-18E, a centralimplant region 2012 can be formed by localized implantation of asuitable n+ species (such as silicon fluoride ions) into the devicestructure as best shown in FIGS. 18C and 18D. The perimeter of thecentral implant region 2012 is a Gaussian surface that defines the innerreflective surface of the waveguide 2002. The penetration depth of thecentral implant region 2012 (which is controlled by the power levelduring implantation) is in the top p-type region (preferably at or nearlayer 1101). During subsequent thermal anneal operations, the n-typespecies of the implant regions 1212 can diffuse to locally shift theband gap in the n-type modulation doped quantum well structure. Thecentral implant region 2012 acts as a barrier to current flow so as tofunnel current flowing from the anode terminal electrode 2051 into theactive region of the waveguide 2002 and away from the central region ofthe device below the top anode terminal electrode 2051. Additionally,implant region 2014 can be formed by localized implantation of asuitable n+ species (such as silicon fluoride ions) into the devicestructure as best shown in FIG. 18D. The implant region 2014 extendsalong the length of the metal 1209 of the first control terminalelectrode 2059 under such control terminal electrode. The perimeter ofthe central implant region 2014 is a Gaussian surface that defines theouter reflective surface of the waveguide 2008. The penetration depth ofthe implant region 2014 (which is controlled by the power level duringimplantation) is in the top p-type region (preferably at or near layer1101). During subsequent thermal anneal operations, the n-type speciesof the implant regions 1214 can diffuse to locally shift the band gap inthe n-type modulation doped quantum well structure. The implant region2014 can be formed along with the implant regions 2012. Additionally,implant region 2016 can be formed by localized implantation of asuitable n+ species (such as silicon fluoride ions) into the devicestructure as best shown in FIG. 18D. The implant region 2016 extendsalong the length of the gap region G between the waveguides 2002 and2008. The perimeter of the central implant region 2012 is a Gaussiansurface that defines the inner reflective surfaces of the waveguides2002 and 2008. The penetration depth of the implant region 2016 (whichis controlled by the power level during implantation) is in the topp-type region (preferably at or near layer 1101). During subsequentthermal anneal operations, the n-type species of the implant regions1214 can diffuse to locally shift the band gap in the n-type modulationdoped quantum well structure. The implant region 2016 can be formedalong with the implant regions 2012 and 2014.

For the waveguide optical coupler of FIGS. 19A-19C, implant regions3017, 3021, and 3023 can be formed by localized implantation of asuitable n+ species (such as silicon fluoride ions) into the devicestructure as best shown in FIGS. 19B and 19C. The penetration depths ofthe implant regions 3017, 3021, and 3023 (which is controlled by thepower level during implantation) is in the top p-type region (preferablyat or near layer 1101). During subsequent thermal anneal operations, then-type species of the implant regions 3017, 3021, 2023 can diffuse tolocally shift the band gap in the n-type modulation doped quantum wellstructure. The implant region 3017 extends along the length of the metal1209 of the top control terminal electrode 3109 under such top controlterminal electrode 3019. The implant region 3023 extends along thelength of the metal 1209 of the top control terminal electrode 3025under such top control terminal electrode 3025. The implant region 3021extends along the length of the gap region G between the waveguides 3005and 3007. The perimeters of the implant regions 3017, 3021, 2023 areGaussian surfaces that define the reflective surfaces of the respectivewaveguides.

Next, a metal layer 1209 (preferably tungsten) is deposited on theresultant structure to interface to the top layer 1107 over desiredimplant regions. The areas where metal layer 1209 interfaces to the toplayer 1107 can form the metal of the gate terminal electrode 51 of theNHFET device (FIG. 13), the metal of the emitter terminal electrode 72of the n-channel BICFET device (FIG. 14), the metal of the anodeterminal electrode portions 81 of the quantum well laser (FIG. 15B), themetal of the anode terminal electrode portions 81 of the thyristor laser(FIG. 17), the metal of the central anode of the closed-loopmicroresonator and the control terminal electrode of FIGS. 18A-18D, themetal of the top control terminal electrodes 3019, 3025 for thewaveguide optical coupler of FIGS. 19A-19C, and the metal of anodeterminal electrode portions 4007A, 4007B of the waveguide opticalamplifier 4003 of FIGS. 20A to 20C. For the optical devices of FIGS.15A-15C, 16, and 17, portions of the anode metal 1209 lies over then-type implant regions 1207 to provide an opening for the aperture ofthe respective optical device. For the waveguide optical amplifier 4003of FIGS. 20A to 20C, portions of the anode metal 1209 lies over then-type implant regions 1207 to define an opening corresponding to thewaveguide region of the device.

Next, the metal layer 1209 is patterned to expose the desired doublelayer oxide/nitride features that underlie the metal layer 1209. Thisstep can provide an offset (or spacing) between the patterned metallayer 1209 and the perimeter of the exposed double layer oxide/nitridefeatures that is on the order of 1 um or less.

Next, with the waveguide regions and/or optical apertures of the opticaldevices protected with photoresist, patterning and etching operationsare performed that expose a first set of mesa regions 1211 preferably ator near layer 1099 (which is above and near the n-type modulation dopedquantum well structure formed by layers 1097-1091). For the n-channelHFET, the mesa regions 1211 are self-aligned to the metal 1209 of thegate 51 on opposite sides of the gate as shown in FIG. 13. For then-channel BICFET (FIG. 14), the mesa regions 1211 are self-aligned tothe metal layer 1290 of the emitter 71 on opposite sides of the emitteras shown in FIG. 14. For the laser/detectors of FIGS. 15A-15C, 16 and17, the mesa regions 1211 are self-aligned to the anode metal 1209 onopposite sides of the active region (which lies between the implants1207) as best shown in FIGS. 15B, 16, and 17. For the closed-loopmicroresonator of FIGS. 18A-18D, the mesa regions 1211 lie outside theclosed-loop waveguide 2002 and inside the zig-zag waveguide structure2001 near the straight section of the zig-zag waveguide structure 2001as best shown in FIGS. 18C and 18D. For the waveguide optical coupler ofFIGS. 19A-19C, the mesa regions 1211 are disposed opposite one anothernear the corresponding straight sections of the waveguides in thecoupling region G as best shown in FIGS. 19A and 19C. For the waveguideoptical amplifier of FIGS. 20A-20C, the mesa regions 1211 areself-aligned to the anode metal 1209 on opposite sides of the activewaveguide region (which lies between the implants 1207) as best shown inFIGS. 20A and 20B. The etching operations that form the mesa regions1211 preferably employ directional plasma etching techniques that formssidewalls that extend downward in a substantially-vertical direction tothe mesa regions 1211 therebelow. Such sidewalls can include thesidewalls 1213 of FIGS. 13-17 and 20A-20B, the sidewalls 2007, 2019 ofFIGS. 18A-18D, and the sidewalls 3041 of FIG. 19C.

Next, an implant of n-type ions is implanted into the first set of mesaregions 1211 to form N+-type implant regions 1215, which are used tocontact to the n-type QW structure (layers 1097-1091) for the device asneeded, such as the n-channel HFET (FIG. 13), the n-channel BICFET (FIG.14), the laser/detector s of FIGS. 15A-15C, 16 and 17, the closed-loopmicroresonator of FIGS. 18A-18C, the waveguide optical coupler of FIGS.19A-19C, and the waveguide optical amplifier of FIGS. 20A-20C. TheN+-type implant regions 1215 can also provide lateral confinement oflight for the waveguide structures as described above.

Next, with the top mesa rib and portions of the N+-type implant regions1215 protected with photoresist, patterning and etching operations areperformed that expose a second set of mesa regions 1217 preferably at ornear layer 1021 (which is above and near the p-type modulation dopedquantum well structure formed by layers 1019-1015). The mesa regions1217 are offset laterally from the mesa regions 1211. For the n-channelHFET, the mesa region 1217 can be offset laterally from the drainelectrode 55 as shown in FIG. 13. Another mesa region 1217 (not shown)can also be offset laterally from the source electrode 55, if desired.For the n-channel BICFET, the mesa region 1217 can be offset laterallyfrom a base terminal electrode portion 73 as shown in FIG. 14. Anadditional mesa region 1217 (not shown) can offset laterally from theopposite base terminal electrode portion if desired. For the quantumlaser/detector of FIGS. 15A-15C, the mesa region 1217 is self-aligned tothe anode metal 1209 on one end of the active waveguide region as bestshown in FIG. 15A. For the quantum laser/detector of FIG. 16, the mesaregions 1217 are offset laterally from the mesa regions 1211. For thethyristor laser/detector of FIG. 17, the mesa region 1217 isself-aligned to the anode metal 1209 on the side of the active waveguideregion opposite the mesa region 1211. For the closed-loop microresonatorof FIGS. 18A-18D, the mesa region 1217 lies outside the closed-loopwaveguide 2002 are best shown in FIG. 18C. The etching operation thatforms the mesa regions 1217 preferably employs directional plasmaetching techniques that forms sidewalls that extend downward in asubstantially-vertical direction to the mesa regions 1217 therebelow.Such sidewalls can include the sidewalls 1219 of FIGS. 13-17, and thesidewalls 2021 of FIG. 18C.

Next, an implant of p-type ions is implanted into the second set of mesaregions 1217 to form P+-type implant regions 1221, which are used tocontact to the p-type QW structure of layers 1019-1015 for the device asneed, such as the n-channel HFET (FIG. 13), the n-channel BICFET (FIG.14), the laser/detectors of FIGS. 15A-15C, 16 and 17, and theclosed-loop microresonator of FIGS. 18A-18C. The P+-type implant regions1217 can also provide lateral confinement of light for waveguidestructures as described above.

Additional directional etching operations can be carried out to define athird set of mesa regions 1223 at the bottom ohmic contact layer 1009for the p-channel HFET device (not shown), the quantum welllaser/detector of FIG. 16, the thyristor laser/detector of FIG. 17, theclosed-loop microresonator of FIGS. 18A-18C, and the waveguide opticalamplifier of FIGS. 20A-20C as needed. The etching operation that formsthe mesa regions 1223 preferably employs directional plasma etchingtechniques that forms sidewalls that extend downward in asubstantially-vertical direction to the mesa regions 1223 therebelow.Such sidewalls can include the sidewalls 1224 of FIGS. 13-17, thesidewalls 2023 of FIG. 18C, and the sidewalls 3043 of FIG. 19C.

Next, the sidewalls of the resulting mesa structure are covered by alayer of silicon oxide (preferably 300 Å to 500 Å in thickness orpossibly thinner), and metal contact areas are defined on the first setof mesa regions 1211, on the second set of mesa regions 1217, and on thethird set of mesa regions 1223. The metal contact areas and oxidecovered sidewalls are then covered by a metal layer 1225. Preferably,the metal layer 1225 comprises a composite metal structure formed bydepositing Nickel (Ni) and Indium (In) metals, which is transformedduring an RTA operation as set forth below into a thermally-stable lowresistance metal layer. Exemplary NiIn composite metal structures can bederived from the deposition of an Ni/Ni—In(xN)/Ni multilayer structure(where each Ni—In layer is formed by codeposition of Ni and In). In thepreferred embodiment, the same composite metal structure is used to formlow resistance metal contact layers to both the n-type and p-typeconduction channels of the integrated circuit. Other suitable metalsand/or metal alloys can be used.

Next, the resultant structure is subject to a wet etchant that removesthe oxide that underlies the metal layer 1225 (on the sidewalls outsidemetal contact regions on the first set of mesa regions 1211, on thesecond set of mesa regions 1217, and on the third set of mesa regions1223) and leaves behind the metal layer 1225 that interfaces to themetal contact regions (on the first set of mesa regions 1211, on thesecond set of mesa regions 1217, and on the third set of mesa regions1223). This step provides a small gap between the metal layer 1225 andthe sidewalls that matches the thickness (preferably 300 Å to 500 Å inthickness or possibly thinner) of the removed oxide. This small gapreduces access resistance to the n-type and p-type modulation doped QWstructures of the device. The wet etchant also removes the double layeroxide/nitride layers adjacent the top anode metal 1209. In order toprevent the anode metal 1209 from etching away in this wet etch, theanode metal 1209 can be protected by a suitable protective layer (suchas thin layer of nickel). An example of a suitable wet etchant is abuffered HF etchant.

Next, it is contemplated that additional process steps, such aspatterning and etching away the top p+ contact layers (layers 1107-1105)and possibly additional layers thereunder in the gap region(s) G betweenthe adjacent waveguides of the evanescent-coupled waveguide devices asdescribed herein, can be performed in order to prevent any chargetransfer across such gap regions. The etched away areas can overlie andpossibly extend into the n-type implant regions in such gap regions G(such as the implant region 2016 of the device of FIG. 18D and implantregion 3021 of the device of FIG. 19C).

Next, an oxide layer is deposited that covers the structure, and theresultant structure is then subjected to a rapid thermal anneal (RTA)operation on the order of 800° C. to 900° C. (or greater). The RTAoperation has two primary purposes. First, it activates all of theimplant regions. Specifically, the RTA can cause the implant regions1212, 1214, 1216 of FIGS. 18C-18D as well as the implant regions 3107,3021, 3023 of FIGS. 19C-19D to diffuse and locally shift the band gap inthe underlying n-type modulation doped quantum well structure (layers1097-1091). This bandgap shift can prohibit charge transfer in the QWsof the n-type modulation doped QW structure (layers 1097-1091) acrossthe gap region G between the adjacent waveguide sections. Voltagesignals applied to the respective top control electrode of the devicecan overcome this effect to allow charge to enter (or exit) from the QWsof the n-type modulation doped QW structure (layers 1097-1091) asdesired. Secondly, the RTA operation transforms the composite metalstructure of the metal layers 1225 to form low-resistance metal contactlayers to both the n-type and p-type conduction regions of the devices.Also note that during the RTA operation, the metal composite layersprovide barrier layers to out-diffusion of the particular ion speciesthat underlies such layers.

In this manner, the RTA-transformed metal layer 1225 can be patterned todefine:

i) the source terminal electrode 53, the drain terminal electrode 55 andcollector terminal electrode 61 of the n-channel HFET device (FIG. 13);

ii) the gate terminal electrode 57, the source terminal electrode 59,the drain terminal electrode 61, and the collector terminal electrode(s)53/55 of the p-channel HFET device (not shown);

iii) the base terminal electrode(s) 73 and the collector terminalelectrode 75 of the n-channel BICFET device (FIG. 14);

iv) the collector terminal electrode(s) 75, the base terminalelectrode(s), and the emitter terminal electrode(s) of the p-channelBICFET device (not shown);

v) the cathode terminal electrode 83 and the collector electrode of thequantum well laser/detector of FIGS. 15A-15C;

vi) the anode terminal electrode, the cathode terminal electrode, andthe collector electrode (which is configured as a floating electrode)for the quantum well laser/detector of FIG. 16;

vii) the n-injector terminal electrode, the p-injector terminalelectrode, and the cathode terminal electrode for the thyristorlaser/detector of FIG. 17;

viii) the electrodes of the closed-loop microresonator as shown in FIGS.18A-18D;

ix) the electrodes of the waveguide optical coupler as shown in FIGS.19A-19C; and

-   -   x) the electrodes of the waveguide optical amplifier as shown in        FIGS. 20A-20C;

Next, an isolation etch down to the substrate is performed in order toisolate the respective devices as needed. Preferably, the isolation etchdown to the semi-insulating substrate 1001 is accomplished by adirectional plasma etching operation that forms sidewalls that extenddownward in a substantially-vertical direction to the substrate 1001therebelow. Such sidewalls can include the sidewalls 1227 of FIGS. 13-17and 20A-20C, the sidewalls 2006, 2018 and 2025, 2027 of FIGS. 18A-18C,and the sidewalls 3045 of FIG. 19C. Note that the isolation etch throughthe bottom DBR mirror can be omitted for the region between themicroresonator and a transistor heater device (or portions thereof) inorder to allow the heat generated by the transistor heater device todiffuse through the bottom DBR mirror to the microresonator for thedesired heating for tuning the characteristic wavelength of the opticalsignal that propagates in the closed-path waveguide of themicroresonator as described above.

Next, the resulting structure can be oxidized in a steam ambient toconvert layers 1003 of the structure to AlO, which form the bottom DBRmirror for the respective devices.

To form an active device suitable for in-plane optical injection into aresonant vertical cavity and/or for in-plane optical emission from theresonant vertical cavity, a diffraction grating (for example, asdescribed in detail in U.S. Pat. No. 6,031,243) can be formed above thewaveguide region of the optical devices as described herein. Preferably,the diffraction grating is formed by ion beam milling of the siliconoxide of the waveguide structure 1201 overlying the active region of therespective optical device.

For cladding for the optical devices, the resultant structure can beprocessed to expose desired areas of the top layer 1107 and one or moredielectric layer pairs can be deposited in such areas of the top layer1107 to form a top DBR mirror 1230 as shown in FIGS. 15B, 15C, 16, 17,18C, 18D, 19B, 19C, 20B and 20C. The dielectric layer pairs form thehighly-reflective mirror of the respective devices. Preferably, thedielectric layer pairs comprise SiO₂ (lower refractive index material)and a high refractive index material such as Si (for wavelengths in theband between 1310 nm and 1550 nm) or TiO₂ (for wavelengths below 1050nm). The top mirror 1230 can achieve high reflectivity by employingmultiple dielectric layer pairs. In the preferred embodiment, six ormore dielectric layer pairs can be stacked upon one another to provide areflectivity on the order of 99.7% or greater.

One or more protective layers can be deposited on the resultantstructure, and a patterned metal layer and via interconnects to theelectrode metal layers 1209, 1225 of the respective devices can beformed through the protective layer(s). This structure can be repeatedfor multiples levels of metallization as is well known in thesemiconductor processing arts.

Advantageously, the self-assembled quantum dots (QDs) embedded withinthe QD-in-QW structures of the optoelectronic devices and integratedcircuits as described herein improves the efficiency of suchoptoelectronic devices and integrated circuits. Specifically, thepopulation inversion necessary for laser action and amplification aswell as the photon absorption mechanism for necessary for opticaldetection occurs more efficiently with the introduction of the quantumdots and thus decreases the necessary current required for lasingaction/amplification and increases the photocurrent produced byabsorption. Furthermore, the size of the embedded QDs can be controlledto dictate the emission/absorption wavelength.

Moreover, the QD-in-QW structures are offset from the correspondingmodulation doped quantum well structure. In one embodiment, such offsetis in the range of 300 to 500 Å as provided by the spacer layers 22 and30 of FIGS. 1-11 (layers 1057 and 1021 in the epitaxial layer structureof FIGS. 12A-12C. By offsetting the QDs from the correspondingmodulation doped quantum well structure, the QDs do not negativelyimpact the performance (i.e., the maximum switching frequency and/orfrequency response) of the QW channel transistors devices realized fromthe device structure, including the HFET and BICFET devices as describedherein. However, the offset can negatively impact the threshold currentand voltage required for certain optical functions (such as lasing andoptical amplification) as well as the response time for opticaldetection. Thus, it is beneficial to minimize the offset to a pointwhere the QDs do not negatively impact the performance of the QW channeltransistors devices realized from the device structure. Note that a biascan be imposed to the collector region of the QW channel transistorsdevices in order to minimize the negative impact of the QDs on theperformance of such devices.

The optoelectronic devices can be formed in arrays of emitters and/ordetectors with associated waveguides and support electronics. Thedetector arrays can have an active imaging architecture or CCDarchitecture. The wavelengths can extend from the infrared band from 850nm to 1550 nm.

The device structure as described herein can also be utilized to realizea single electron transistor (SET). The SET is a building block used inquantum computing that makes direct use of quantum-mechanical phenomena,such as superposition and entanglement, to perform operations on data.In quantum computing, a qubit (or quantum bit) refers to a unit ofquantum information—the quantum analogue of the classical bit. The qubitis a two-state quantum-mechanical system, such as the spin state of asingle electron: here the two states are the spin up and spin downstates of the electron. In a classical system, a bit would have to be inone state or the other. In contrast, quantum mechanics allows the qubitto be in a superposition of both states at the same time, a propertywhich is fundamental to quantum computing. Quantum computing can be usedto carry out quantum mechanical (QM) algorithms that execute particularcomputation intensive problems. For example, Shor's algorithm waspublished in 1994, and shows that the problem of integer factorizationis substantially faster when run on a quantum computer than when usingthe most efficient known classical factoring algorithm.

The SET confines an electron with a predetermined initial spin state(i.e., spin up state or spin down state) within a small volumesurrounded by a potential barrier. During a load operation, an electronis loaded from a source electrode into the small volume via tunnelingthrough the potential barrier between the source electrode and the smallvolume. The electron is then isolated for a period of time (computationperiod) within the small volume where no attempt is made to query thespin state of the electron (i.e. no energy is removed). At the end ofthe computation period, a read operation determines the final spin state(i.e., spin up or spin down) of the electron by attempting to read outthe electron from the small volume to a drain electrode via tunnelingthrough the potential barrier between the small volume and the drainelectrode. From this description, it is evident that the SET provides agateway between a quantum mechanical system and the macroscopic world.It is well known in physics that the true state of the variables in aquantum mechanical system can never be monitored in real time becausethe act of measurement forces the quantum mechanical system to be in onestate or the other (i.e. to be deterministic) which by definitionprevents the natural evolution of the quantum mechanical system in whichthe variables are not digital but instead consist of a certainprobability of being in several different states simultaneously. Howeverat some point (i.e., the end of the computational period, the data mustbe transferred to the macroscopic world where conventional computingprinciples can be applied.

FIGS. 21A and 21B illustrate an exemplary embodiment of an SET realizedfrom a device structure similar to that described above with respect toFIGS. 12A-12C. The SET is similar in structure to the NHFET device ofFIG. 3 with a gate terminal electrode 1301, a source terminal electrode1303, and a drain terminal electrode 1305. The source terminal electrode1303 and the drain terminal electrode 1305 of the SET are operablycoupled to opposite ends of a QW channel(s) realized in the n-typemodulation doped QW structure of layers 1097-1091 by correspondingn-type source and drain implant regions 1307, 1309. A QD structure 1311is formed in the device structure and covered by the gate terminalelectrode 1301. The QD structure 1311 achieves confinement in thevertical (z) direction by the n-type modulation doped quantum wellstructure of layers 1097-1091. The QD structure 1311 is preferablyrealized by a single QD incorporated with the device structure, whichcan be isolated from other devices by etching down to the p+ layer 1101.To create the QD, a photomask is used to define an opening for ionimplantation with the pattern of a ring or square. The internal diameterof the opening is about 100A (10 nm) and the width of the opening isalso about 100A (10 nm) and defines the x and y dimensions of the QDwhile the quantum well defines the z dimension of the QD. Oxygen ions(i.e., O−) are implanted into the device structure through thering-shaped photomask. The depth of the peak of the ring-shaped oxygenion implant 1310 is centered over the N+ charge sheet 1097 of the n-typemodulation doped quantum well structure of layers 1097-1091. The oxygenion implant density is controlled such that the RTA cycle of the devicestructure causes the oxygen ions to react with the N+ doped charge sheetand convert it to high resistance. In one embodiment, the N+ chargesheet 1097 of 15% AlGaAs is doped to a level of 3.5×1018 cm⁻³ and theoxygen ions are implanted to a density of at least 1.75×1019 cm⁻³. It isexpected that subjecting the sample to an RTA cycle of greater than 800°C. for 10 secs will be sufficient to convert the N+ charge sheet 1097 tohigh resistance. By converting the N+ charge sheet 1097 to highresistance, the threshold voltage over the width of the ring-shapedimplant region will increase to >2V and the surface potential isessentially zero. Since the surface potential inside the ring-shapedimplant region 1310 is about 1V, a QD is defined inside the ring-shapedimplant region 1310 with dimensions of 60×100×100A with a well potentialof approximately 1V. The ring-shaped implant region 1310 confines andisolates the electron in this QD during operation of the SET. Thus, a QDstructure 1311 is defined internal to the SET with a potential barrier(labeled “Source Barrier 1313”) of 100A thickness between the sourceterminal electrode 1303 and the QD structure 1311 and a potentialbarrier (labeled “Drain Barrier 1315”) of 100A thickness between the QDstructure 1311 and the drain terminal electrode 1305. The gate terminalelectrode 1301 can be formed on the p+ layer 1101 similar to the gate ofthe n-channel HFET device. In this manner, the gate terminal electrode1301 is formed over the QD structure 1311 and over both the source anddrain barriers 1313, 1315. Since the gate terminal electrode covers theQD structure 1311, the voltage level of the gate terminal electrode 1301controls the energy level in the QD structure 1311. However, the voltageof the gate terminal electrode 1301 will have little or no effect on themagnitude of the source and drain barriers 1313, 1315 because the N+charge sheet 1097 has been converted to high impedance. The voltagelevel of the gate terminal electrode 1301 with respect to the sourceterminal electrode 1303 can be controlled to allow for tunneling of asingle electron through the source barrier 1313, and the voltage levelof the gate terminal electrode 1301 with respect to the drain terminalelectrode 1305 can be controlled to allow for tunneling of the electronthrough the drain barrier 1315 during the read operation.

The operation of the SET of FIGS. 21A and 21B is illustrated in theschematic diagram of FIG. 22A and the accompanying waveform diagram ofFIGS. 22B and 22C. During the load operation, if the top energy level inthe QD structure 1311 is aligned with the Fermi energy of the sourceterminal electrode 1303, electron tunneling can occur through the sourcebarrier 1313 into the QD structure 1311 if the top energy level in theQD structure 1311 is empty. However, the top energy level in the QDstructure 1311 is split into two levels by the Zeeman effect due to thepresence of a large static magnetic field B with spin up being at ahigher energy than spin down as shown in FIG. 22A. The static magneticfield B can be generated by an external permanent magnet, anelectromagnet or other suitable device. The corresponding electricalfield E is also shown. Therefore the Fermi energy of the source terminalelectrode 1303 can be set to an appropriate energy to dictate apredefined spin state (e.g., initial “spin up” state). In this manner, abias voltage level of the gate terminal electrode 1301 with respect tothe source terminal electrode 1303 can be applied to enable electrontunneling through the source barrier 1313 into the QD structure 1311 aswell as initialization of the spin state of the electron in the QDstructure 1313 to the desired predefined “spin-up” state. This isperformed during the load operation as illustrated in the exemplarywaveforms of FIGS. 22B and 22C. It is assumed that the QD structure 1311is small enough and the electron wavelength is large enough that onlyone electron at a time can tunnel. Therefore only one electron is loadedfrom the source terminal electrode 1303 into the QD structure 1311.

The compute operation follows the load operation. Specifically, once theelectron has tunneled to the QD structure 1311, the electrostaticpotential of the QD structure 1311 is move down (negative charge) bylowering the bias potential applied to the gate terminal electrode 1301as illustrated in the exemplary waveforms of FIGS. 22B and 22C. Thelower electrostatic potential of the QD structure 1311 limits theability of the electron to tunnel back to the source terminal electrode1303. Thus, during the compute operation, the electron is confinedwithin the QD structure 1311 where it is isolated for a period of time(computation period) with no attempt made to query the spin state of theelectron (i.e. no energy is removed).

The read operation follows the compute operation. During the read cycle,a bias voltage level of the gate terminal electrode 1301 with respect tothe drain terminal electrode 1305 can be applied to enable electrontunneling from the QD structure 1311 through the drain barrier 1315 tothe drain terminal electrode 1305. Specifically, such bias voltage isset to allow for tunneling of electrons that have the spin up energystate (while not allowing tunnel of electrons that have the spin downenergy state). Thus, if the electron has the spin up state, the readoperation will produce a minute current pulse output as shown in FIG.22C. However, if the electron has the spin down state, no current outputwill be observed during the read operation. Note that in order to bepractical, the thermal excitation current must be quite a bit less thanthe tunnel current. Therefore the tunnel thickness must be reduced andthe barrier height increased sufficiently until this condition is met,for otherwise cooling will be required

Although the SET as shown employs a single QD (which we can recognize bydefinition as being synonymous with a single qubit), it is clear thatmultiple QDs (i.e., qubits) with a source electrode input and a drainelectrode output may be constructed with controlled tunneling barriersbetween the QDs. Furthermore, the QDs can be extended in both the x andy directions according to the implementation of an arbitrary quantumalgorithm. The gate may be common to all the qubits or group of qubits,i.e. multiple gates may also be used. Thus, an arrangement of SETs maybe constructed according to the design required for a specificalgorithm, such as the Shor algorithm.

Advantageously, the SET of the present application allows forrealization of electron spin state as a quantum mechanical variable insuch a way that the SET is also compatible, in the same technologyinfrastructure, with conventional state-of-the-art logic circuits. Thisallows the inputs and outputs from SET quantum computing function tosupport and take advantage of the existing methodology for highperformance computing.

There have been described and illustrated herein several embodiments ofan optoelectronic integrated circuit employing quantum dots embedded inone or more quantum wells and a method of fabricating the same. Whileparticular embodiments of the invention have been described, it is notintended that the invention be limited thereto, as it is intended thatthe invention be as broad in scope as the art will allow and that thespecification be read likewise. Thus, while particular group III-Vmaterial system and heterostructures have been disclosed, it will beappreciated that other III-V material systems and heterostructures canbe used to realize the optoelectronic integrated circuitry as describedherein. It will therefore be appreciated by those skilled in the artthat yet other modifications could be made to the provided inventionwithout deviating from its spirit and scope as claimed.

What is claimed is:
 1. A transistor device comprising: a gate terminalelectrode disposed between a source terminal electrode and a drainterminal electrode; wherein the gate terminal electrode overlies aquantum dot structure embedded in a quantum well of a modulation dopedquantum well structure that includes a charge sheet offset from thequantum well, and wherein a potential barrier surrounds the quantum dotstructure.
 2. A transistor device according to claim 1, wherein: thepotential barrier is defined by an ion implant region that surrounds thequantum dot structure.
 3. A transistor device according to claim 2,wherein: the ion implant region is formed from an ion species thatreacts with the charge sheet of the modulation doped quantum wellstructure under predefined high temperature conditions.
 4. A transistordevice according to claim 3, wherein: location and dimensions of thepotential barrier are dictated by the location and size of an openingdefined by a photomask that allows for the implantation of the ionspecies.
 5. A transistor device according to claim 3, wherein: the ionspecies comprises oxygen ions.
 6. A transistor device according to claim1, further comprising: means for biasing the gate and source terminalelectrodes to allow for tunneling of a single electron from the sourceterminal electrode through the potential barrier surrounding the quantumdot structure and into the quantum dot structure; and means for biasingthe gate and drain terminal electrodes to allow for selective tunnelingof a single electron from the quantum dot structure through thepotential barrier surrounding the quantum dot structure to the drainterminal electrode, wherein the selective tunneling of the singleelectron is based upon spin state of the single electron.
 7. Atransistor device according to claim 1, wherein: the quantum dotstructure is self-assembled.